My DD was fully toilet trained at 20 months aswell, all of her own accord, I just followed her lead. Anyway, she has regressed a few times, even this week she's had a couple of accidents, I just go with it as it usually only lasts a few days and then she's back to no accidents again. In our case I think DD holds on for too long and then simply hasn't got time to get to the toilet! This week particularly, she actually starts going in her knickers while running to the toilet, so it really is a case of holding on till the very very last minute!
I definately would not put her back in nappies, I think thats giving the wrong message but thats just IMO of course

I would just just roll with it and reinforce and encourage use of the toilet.