Did you recently change your set up, moving seat higher, flipping your post or something else that would lead to a more aggressive position? If not, and you usually don't have problems at that distance, it could be tight hamstrings, strained muscles in your hip girdle, weak core, etc. You could also have strained something back there either on or off the bike. See how you feel tomorrow, and maybe start doing stretching before getting on the bike in the short term, and core strengthening/flexibility exercises in the long term.