We have been going through Hell with Thomas. Poor guy has been withholding his stool for weeks. What I thought was him "ready to move to underwear" was just an unsuspecting constipation and pulling on his diaper. I potty trained him in a few days, but after refusing to go #2 for several days we knew we were in trouble.
We've had multiple bowel clean outs, trips to the ER, and an endoscopy too (unrelated to the withholding, but symptomatic of the problem). We start therapy this week.
Please say a prayer we get through this period quickly. Poor baby is back in diapers and completely miserable.