Have you considered maybe a hiatal hernia ? It is when the top of the stomach bulges up through the diaphragm. It can come from straining efforts.

I have had hiatal hernias. They can cause the stomach pain you describe. The doc taught me to resolve mine by doing heal drops. On a solid floor, concrete or such, rise on your toes and drop your heals to the floor. The jar can pull your stomach back into place. In some extreme hiatal hernias, surgical intervention is required. They can do it with a laparoscope. Ask you doctor about your symptoms if the heal drop does not help.

My chronic TOS can also cause stomach trouble. My physical therapist helps me with that.

You may be putting too much effort into your exercise.

In have a friend who is currently going through the gall bladder pain problem. He does not have gall stones. He has a 90% clogged bile duct and weak wall to his gall bladder. When his gall bladder needs to release bile, very little is released. A ruptured gall bladder can be a serious issue. There are some imaging systems that can diagnose these problems.