Feels like I'm carrying a bag of rocks!!

Hi there i'm 23+2 and i dont know if its the position the baby is but i swear sometimes all of a sudden he feels like a 20 lbs bag of rocks!!! I have a petite/athletic figure so i knew back pain and all would kick in early but gosh baby can feel heavy no matter if I just ate or anything. OB never told me he was bigger than average so i guess its just how i carry. Any advice for the next 17 Weeks?? I wear a belly belt when i go for a walk but other than that...