Sakit perut mcm tu common je at 20 weeks sebab baby growing dan ligaments stretching, tapi since kadang-kadang rasa very painful better check with your doc just to be sure everything's okay, a few of my mum friends had similar cramps at that stage and it was fine, but some also needed reassurance from their gynae. The gas and constipation thing very common in pregnancy jugak, so maybe try drinking more water dan makan high-fibre foods, tapi jangan main-main, book an appointment to get checked out okay.