There are many causes of right-sided abdominal pain, some of which are serious and others are very benign. If the pain is bothering you, I suggest that you see a doctor who would have the advantage of doing a physical exam to figure out what the cause of your pain is. When you do make an appointment, be prepared to discuss the following possibilities.
If your pain started directly after a work-out, it is likely musculoskeletal in nature. You may have a small tear in the abdominal wall musculature that was caused by strain during your work-out. This pain would be worsened by flexing or coughing as you describe. The best treatment for this type of injury is to avoid strenuous activity for a short period of time and to take over-the-counter pain medications. Tylenol or ibuprofen would both be good choices.
Another cause of right lower quadrant pain can be appendicitis, but this pain is usually a deep pain that is constant. Appendicitis would also be accompanied by other symptoms such as nausea, vomiting, fever, and anorexia. The pain in appendicitis would be worsened by coughing or moving, but would likely not go away by sucking in your stomach.
A final cause of periumbilical pain could be enteritis or colitis. When the small intestine or colon becomes inflamed, it can cause pain on either the left or the right side of abdomen. This pain is usually crampy rather than sharp, and can be associated with nausea and diarrhea. If you have GI symptoms like these with your pain, enteritis/colitis may be the cause.
It is not possible to diagnose the cause of your pain without your seeing a physician. I suggest you make an appointment with your primary care doctor
promptly so that he or she can evaluate your and order any necessary further tests. If appropriate, you can be referred to a gastroenterologist
or a surgeon