pains in stomach

Currently 26weeks and 4 days. Today I went shopping with my mom and after walking around for awhile my stomach started to hurt. It was kinda in the middle but felt like it was going around my stomach. I've felt my son move alittle bit today but not as much. Should I be worried? I had an ultrasound done on Tuesday and was told he is perfectly healthy. I've also had really bad heartburn and acid reflux today. Could that be the reason why?