More on Ben's hip pain:

I've been on the internet looking up a few things regarding Ben's hip pain complaint yesterday.
I should specify that Ben has not been diagnosed with anything, he simply complained of hip pain and developed a fever. He was not limping, and he never lost the ability to walk. He simply got tired, and I asked him to lay down. The Urgent Care doctor told Dad that this sort of pain ws common in toddlers, and suggested we watch for a developing limp or continued fever or pain complaints. He's fine today, and says nothing hurts. I've been hounding him all morning. Nevertheless, I thought I'd do my homework. Based on the articles I've read this morning, I think I'll suggest bloodwork next week at our family practioner, just to make him aware of things. I thought I'd pass on some of what I read this morning.

This article I found on the Parenting ivillage website stood out to me b/c Ben had a sinus cold recently:

"Toxic synovitis also known as acute transient synovitis of the hip is a condition in which there is inflammation and fluid within the hip joint. It is this swelling and fluid accumulation which causes joint pain. It is a rather common problem with up to 3 percent of children having an episode at some time during their life and may occur in children anywhere from the early toddler to the teenager. However, it usually happens to kids between 3-6 years of age. The exact cause of toxic synovitis is still unknown. The only thing that seems to be consistently associated with toxic synovitis is that many of these children have had an upper respiratory illness shortly before the onset of hip pain. This has led to speculation that the virus which is causing the cold also causes the synovitis. No matter what the cause, almost all children recover within two weeks and without long-lasting effects. Between 4-17 percent of children may have a second episode of toxic synovitis, but this usually occurs within 6 months of the first episode. Treatment for this disorder focuses primarily on rest and taking anti-inflammatory medications. Rest usually consists of limiting the amount of walking done. Bedrest may be required at first to get rid of the pain, but then more use of the hip can be done gradually as the pain subsides."

The emedicine website calls this condition "Transient Synovitis." Here's what they have to say:

Frequency:
In the US: Little data are available regarding the frequency of this illness. However, excluding infections and trauma, TS is one of the most common causes of joint pain in the pediatric age group.
Background: Transient synovitis (TS) is the most common cause of acute hip pain in children aged 3-10 years. The disease causes arthralgia and arthritis secondary to a transient inflammation of the synovium of the hip.
Sex:
TS affects boys twice as often as girls.
History:
Hip pain: Unilateral hip or groin pain is the most common report; however, some patients with transient synovitis (TS) may report medial thigh or knee pain.
Crying at night: Very young children with TS may have no symptoms other than crying at night; however, a careful examination should reveal some degree of an antalgic limp.
Recent infection: Recent history of an upper respiratory tract infection, pharyngitis, bronchitis, or otitis media is elicited from approximately half of patients with TS.
Limp: Some patients with TS may not report pain and may present with only a limp.
Fever: Children with TS are usually afebrile or have a mildly elevated temperature; high fever is rare.

Here is some more info I found on Dr. Greene.com

Toxic Synovitis
Related concepts: Transient synovitis, Postinfectious arthritis
Introduction: Whenever children develop a limp they should be checked to be sure that the cause is not something that needs emergency treatment, such as septic arthritis. When the results come back, parents will often hear that the diagnosis is toxic synovitis. This sounds like bad news, but it is good news.
What is it? Toxic synovitis of the hip is the most common form of arthritis in children. It appears suddenly, disappears suddenly, and causes no lasting problems. Thus, it is often called transient synovitis.It typically follows viral infections, and may be caused by the body’s immune response to the virus. For this reason, it is often called postinfectious arthritis. Many viruses can lead to some type of postinfectious arthritis --
Who gets it? Toxic synovitis can happen in any child, but it is most common in boys between the ages of 3 and 10, after an upper respiratory tract infection.
What are the symptoms? Boys usually complain of pain in the hip, thigh, or knee. They suddenly develop a noticeable limp. There is usually no fever, redness, or swelling of the joint.
Is it contagious? No. The viral infections that lead to toxic synovitis are often contagious.
How long does it last? Toxic synovitis usually disappears completely within a few days.
How is it diagnosed? Children with a new limp usually need to be examined, have a blood test, and an imaging study to be sure there is not a condition that needs emergency treatment. In toxic synovitis, the blood tests are usually normal.
How is it treated? Toxic synovitis needs no treatment other than pain relief.
How can it be prevented? Usually toxic synovitis is not preventable, except by avoiding the viral infections that can trigger it.