It sounds like sciatica. The tennis ball idea is good and you can do it a few times a day and it helps when you actually have the pain too. Otherwise if you get on all fours, then straighten one leg and cross it behind the other and gently push back into the stretch, that may help. I seem to remember doing that one in prenatal. I get this a lot but the best exercises I have found for this are difficult with a big belly but hopefully the ball and this stretch will give you some relief.

ETA otherwise I just looked on a yoga site and it said if you stretch the other muscles in your legs like the calf then it may help too.