How can i become more flexable? Does running ruin flexablity because it tightens your hamstirings??

I did pilates and i was pretty good at it (from the start i was pretty flexable) and used to do some cardio as well... bt then i stopped doing pilates and started running heaps... now im bak to pilates (nd running at the same tym) bt i fynd i am much less flexable than when i used 2 do it. Is this because my hamstrings have tightened a lot? (I have more muscler legs nd thighs now from running) Have hamstrings got anything to do with it?? Nd how can i improve my flexability anyway?? because i want to eventually be able to do the splits.