Potty regression...?

N has been potty trained for well over a month now. What started with him refusing to put a diaper on, turned into him running around naked and using the potty successfully. Then we introduced underwear and braved leaving the house in them. He has been so great and had very few accidents. But the cold that he got last week and the messed up sleep schedule seem to have caused a regression with using the potty. When I saw his pediatrician the other day, she was first totally amazed that he's using the potty and then said it was totally normal to regress when sick and that he'd get back on track when he started to feel better. But as I picked poop up off the carpet this morning I had my doubts and even wondered if he needs to go back into diapers. He's stopped telling me that he needs to go and if I ask him or take him to the potty he says no! Today I asked him if he wanted to wear a diaper or his 'big boy underwear' and he picked the underwear so we stuck with that and he was clean and dry for the rest of the day.

I'm just not sure what to do. I don't want to confuse him or take away from what he's done but also don't want to be cleaning up accidents all the time, or feel like I'm watching him like a hawk for signs he needs to go. I'm not sure if it's because he's been sick, or because the novelty has worn off. Or if he's just too busy with other things.... His language has really taken off and he now uses 2 or 3 word sentences and is constantly adding new words.

Because he led the way with the whole potty thing I never used any kind of rewards - stickers, candy, prizes etc. And never have with any of the kids I've potty trained over the years. I also don't use pull-ups as I think they are the same as diapers and too confusing. At this point he only wears a diaper at night, he stays dry during naps, and is sometimes dry in the morning too.

He's only 20 months old and his pediatrician said that most kids aren't potty trained until they are 3. Most kids that I've taken care of have been potty trained between 18 months and 2.5yrs so I guess I'm going to give him a bit longer and see what happens.