Sounds like sciatica to me. I had this from about 12 weeks and still have it now. I use a heat pack and try to stay off my feet or if that's not possible then try to just move around slowly as I know when it really pinches it can send you to your knees.
I tried physio, chiro etc etc and the heat is the only thing that really helped. I've been told the only thing that fixes it is giving birth!
It can be pretty painful so hopefully it goes away for you soon!!