I had the same problem and it turned out that my body was lacking nutrients. I was put on extra iron supplements and had to take extra vitamins. They stopped once I figures out what I was missing.
They just started a few nights ago I just thought they were cause I'm close to my due date:(
I would definitely check in with your doctor. Pregnancy causes so many changes with our body lies it could be a number of things and is probably (hopefully) an east fix! You shouldn't have to suffer through them every night. As a migraine sufferer for over 16 Years (I'm only 21!) I know how dabilitating they can be. I miscarried about a month ago but funny thing is I had significantly less migraines while pregnant! It could be a hormone thing so it wouldn't hurt to get it checked out. Good luck mama!