I hope it's not a big problem
I know Fred Hemmes injured his stomach muscle in January and he just started to play again this week. Played qualies and finally had to retire in the second round of MD. Can't find the reason, but I think there's a good chance it's his stomach muscle again. And before his "break" he's been out for a few months, because he injured some muscles between the ribs. So as Takuma said, it could be anything and the time to heal depends on the injury.
If it looks like I'm a walking Fred encyclopedia, I'm not, I'm just very good at remembering useless facts, especially the ones I put on my website