A pulled or tired muscle, a bruise or swelling due to a minor injury, a minor infection which has caused the lymph nodes in the area to swell, etc. There is no shortage of possibilities. Most are minor, some possibly serious (but those are rare and unlikely).
If you have difficulty walking, or the pain has gone on for a few weeks, or the pain gets worse, or other symptoms appear (such as a rash), or you're simply worried, see your GP.
