I have had central pain, which feels like a deep pulled muscle. The right side of my stomach always sticks out a little more than my left, and I've been having an issue sucking in my stomach as much as I used to. I've been having issues with gas, bloating and constipation though and I do notice that after a successful washroom trip I can suck in my stomach a bit more, as gross as that sounds. If you've been working out more than usual it can definitely be tenderness from that. If I do a lot of sit-ups and crunches my stomach feels/appears a bit more distended because I naturally suck in my stomach when I walk, and the muscles are a bit too tender to allow me to. I would suggest heading to a clinic and let the doctor have a feel for any masses and get some input on what's causing the discomfort. I wouldn't be overly worried but nothing beats peace of mind!