Sounds like a very minor strain or tightness possibly brought on by scar tissue from a previous injury.
Get yourself a foam roller and do this
http://www.youtube.com/watch?v=WErpKRJFQ3A on both hamstrings.
Also get yourself a hockey or tennis ball (I prefer hockey as it gets in deeper) and do the same thing concentrating on the area you have identified to be the problem during the foam rolling (note, it will be f*cking sore). Take a week or two off football and do this everyday (or twice if you can find the time). Important to keep using the muscle during recovery so you can go for a very light cycle on a stationary bike or something like that (note, when I say very light, I mean very light, you shouldn't be out of breath at all and resistance should be on the lowest setting). Plenty of stretching too.
Go back to football but continue with the above for a few weeks as well (minus the bike), and see how you feel. If it's still bothering you, go and see a physio for a full assessment (it could be coming from lower back, changes in your gait, multiple things and without anyone seeing you in the flesh these things won't be found out).
Good luck! Let us know how you get on with it!