Diaper rash - teething or diapers?

The only time DS has had diaper rash was when he was on antibiotics for an ear infection. Otherwise, he's rash free. Today I noticed he's getting what seems to be a pretty uncomfortable bout of it. I'm trying to figure out what it could be from. We have exclusively been using Honest Co diapers since he was born, but he's a tummy sleeper and has been leaking, so a few days ago we started using Huggies Overnights for just his nighttime diaper. He's also teething right now, with another tooth just about to pop through. Which is more likely to be the cause - new kind of diaper just once per day or teething? He doesn't have diarrhea, but has been pooping more often.