hi, as the gestational age progresses uterus will grow and becomes upright from its anteverted position, because of these it will exert pressure on surrounding bowels leading to feeling of bowel movement. better to stop soda instead you can go for some fruit juices, for gastritis you can take antacids, belly small means probably abdominal circumference is less, if severe it may indicate intra uterine growth retardation, in mild cases no need to worry, it will get corrected with time, try to take adequate and proper diet. take care.