Is it common/normal to start having lower back pain at 27 weeks?

[Baby Lily July2]

I will be 28 weeks tommorrow. The past few days or so Ive been having lower back pain, and yesterday all day whenever I urinated it burned and stung. But today it didnt at all. But I still have lower back pain sometimes, and the kind of pain it feels like is not joint or bone, but it feels like I have water in my kidneys or something, I know it's hard 2 explain. It's a heavy feeling in my back lower back and also my abdomen sometimes, a heavy full feeling not pressure but not bloating either. I have been a little constipated lately, could this be why? today I had a little bit of diahrreah (sorry 2 be so graphic). Is this normal? I dont feel bad otherwise and the baby is moving like normal. And I'm not high-risk. This is my 1st pregnancy.