tight hamstring

i did an hour on my turbo trainer last night and for some reason my right hamstring felt really tight as though it was going to cramp up all the time. i wasnt really pushing it hard and my left one was ok and still had alot of energy left. any ideas on what could cause this. it started happening about 20 mins into the sessions.
2010 Forme Reve
2010 Giant Talon 1
2010 Giant Talon 1
0
Posts
For me its due to having very tight hamstrings, so i need to a good routine of stretches a couple of times a week to stop them tightening up and pulling..
http://www.grouptrails.com/Stretches.htm
Hope this helps...