Is it after moving quickly? Your baby is growing so fast at the moment it could be ligaments. Also uti's are very common in pregnancy and can cause cramps in back and pelvis. Go to your docs if you've not been checked recently. Best of luck x
Try drinking a glass of water and laying down. If they hurt a lot then you can always go to L&D to be checked. And back pain is also usually normal.
And my back is killing me