I had this problem too and when i went to a trainer he found that it was due to inflexibility. It's gotten a lot better through stretching my hamstrings prior to lifting and rolling out my thighs with a foam roller. I also found that yoga helped a ton too! Good Luck!
Me too! I started the 5x5 method after my boyfriend pointed it out to me on Crossfit. After doing this program for a month, 2x/week, I've started to increase my the weight I'm lifting as well as see the added benefit of leaning out!