Plaidophile

Whenever I have an inexplicable pain, I treat it with either Aleve™ or a combination of aspirin, acetaminophen and coffee. (For the latter solution, you can also just buy Exedrin³™, but I'm a tightwad and the coffee tastes better.) If it doesn't go away after a week to ten days, I make a trip to the doctor.

If I know why I hurt, the time period before I visit the doctor generally gets longer, but will occasionally get shorter. The key factors being the why and the what.

But as for causes, my guess would be that either you stayed in a position that you're not accustomed to for an extended period of time or you engaged in physical effort in a manner you aren't accustomed to.

Holy product placement, Batman!

I haven't done anything differently than usual.

If it's a blood clot, waiting to see a doctor could prove fatal.

It's not a blood clot. A blood clot would be accompanied with feelings of numbness and/or sharp stabbing pains. This feels more like a pulled tendon.

Is the pain localized along the side of your knee (either side)? If so, it could be illiotibial band syndrome, which is usually not severe, but occasionally (like in my case) either refuses to go away or morphs into something more severe.

Unfortunately, all the doctors have been able to give me for it is naproxen (moderately effective) and dexamethasone (works great, but being a steroid, is troublesome in myriad other barfulous ways).

No, it's up in the pelvic region. Though now it's gone.

It's a tooomor!

Oh, is THAT what that growth is?

Was the pain around a joint area and slightly tender (like an inflammation?)

Joint area, yes, wrong on all other counts.