I'm no expert but it maybe siatica! (Not sure if I spelled that correctly)
It is quite common in pregnancy and can be quite painful. I suffered from it and it probably started around 25weeks..It was on my right hand side just below my hip bone.

All the pressure on your nerves in your back from bubs and everything growing and stretching!

Maybe someone else might have a suggestion but I would just go and get it checked just to ease your mind!!