I agree w/adgal....I had the same pain for a wk or 2. Your body will adjust in a short time. But you may feel the same discomfort after wk 20 at some point. Congrats on your first trimester being OVER!
Baby is growing, and chances are its just ligament stretching pain. Your body making room so to speak. However, its always best to be safe, and you should give your Dr. a call. Any cramping or pain really should be reported. I'm sure all is well, but please give your Dr. a call. Best of luck to you, and congrats on your pregnancy, and being through the first trimester!!!