Pre-pregnancy my left side pooched more than my right when I leaned back (standing) and looked down. As a nurse, I figured that's because the large intestine/colon wraps around that side before going down. 17 weeks pregnant, I've noticed the pooching is higher up because the uterus has grown.
I'm sure it's most likely just where the baby has settled and all is well but have you been checked for any uterine abnormalities? Often they aren't diagnosed unless you have a history of miscarriage and they are looking. I have a partial septate uterus (meaning my uterus is a deep heart shaped rather than pear) and my baby is firmly lodged on the left side, making my bump quite obviously lopsided. It might be worth asking about at your next ultrasound just for peace of mind, as I have to be monitored more closely for preterm labor. Again, it's super rare and chances are you're totally normal but just throwing another possibility out there!
Re: Lopsided