It could be a couple of things. While your pregnant your digestion slows down, so it can take longer to process food that can lead to discomfort. Your body is also preparing for a growing baby so it's trying to stretch muscles that haven't been used before, or not very often.
Some of the medication that you are probably taking might make you feel constipated and some may have as a side effect gas! That can be an option too.
Thanks for the advice ladies