9 month old keeps having diarrhea

FTM. My 9 month old son pooped 4 times yesterday and today he's had 6 diarrhea diapers! His poop is green if that means anything. A couple weeks ago I switched him from breast milk to formula so maybe that could be a cause for all of the poopy diapers the past few days? He has a bad diaper rash now and I feel so bad about it because he cries whenever I wipe his butt. I've been smearing diaper rash cream on him and changing diapers frequently. What can I do for him to help his pain and clear up his rash fast? Does this warrant a Dr call?