Please listen to me - stomach distension should ALWAYS be checked by a medical professional. One of my kids had slightly distended stomach which we attributed to a big rib cage and still needing to grow into his bigger frame.
It was a mature teratoma weighing over 20lbs
He had a physical with no issues just three days before we visited the ER for stomach pain. ER found it immediately when they noted the stomach distension during his exam and did scans to find out why.
Maybe it’s nothing. Maybe it’s gas. But also maybe you devoured your twin in the womb and he’s back for revenge.
Please see a doctor and ask for an ultrasound at the very least.