Increase in Bowel Movements With Straining

Kristen
OK so my little one has always pooped usually only once, maybe twice daily. But now all of a sudden he's pooping constantly. I mean like once every hour. And for a lot of these pooped I noticed he strains relatively hard to get it out. Sometimes I even have to manually massage his wrecked him to help him relieve himself and get it all out. Also, each poop is very small and yellow in color. It's not runny but it's also not super solid. He was on Similac Spit Up but I recently switched him to Emfamil Gentle Ease which he was on for roughly a month while he was in the NICU just last month. So I know he tolerates it just fine because she had no problems when he was drinking it then. So I'm at a loss of what's causing this sudden increase in bowel movements. The frequency it is causing him to get a diaper rash as well because of having to wipe him so often. I always put diaper cream on him to keep it under control so I do everything I should but it's just another reason that this constant pooping is an issue. Does anybody have any thoughts, guesses, experiences, or answers to this issue?