Did you maybe eat sonething that has you bloated? Whenever I bloat I feel it in my back and not my stomach. If it's really bad though, call your doc. No harm in asking.
Have u gained alot of wieght?it could be just that ur stomach is to heavy im 21 weeks im bearly starting to get back pain too take a warm bath and just rest a little or have somebody give u a massage try laying on ur side