Sounds muscular, but you didn't mention nature of pain man - is it constant or throbbing/pulsing? Is it there all the time or just when you move? Presuming you didn't knock it in the last few days or would have mentioned.....can you see any bruising/contusion? Is the area tender to touch - if muscular injury, it should be, but that might also be indicative of contusion.
Check out this site - pretty good
http://www.nhsdirect.nhs.uk/
Ultimately, I'd say if it persists, bite the bullet and just go see your doctor - in the meantime, take an analgesic - it will relieve muscular inflammation - ibuprofen if you it - I'd not recommend a strong painkiller as may hide symptom (pain) that could be something else.
Go see doctor if concerned man - and do check out that website in the meantime : )
Rich