Unbearable PAIN in lower leg.?

About twice a week I wake up in the middle of the night with such an unbearable pain in the lower part of my legs. Sometimes it's the right leg. And sometimes it's the left leg. What usually happens is I will wake up to use the bathroom in the middle of the night and go to stretch my legs and one of them will lock and I get this excruciating pain for about 15 seconds where I CANNOT move my leg at all to release it. It's literally so painful I wake up my fiance and I'm in tears. I don't have insurance right now so anybody with any suggestions or any knowledge of this would feel free to share! Thank you!