I need parenting help ASAP. :)

Lily, who up until now has been a world-class napper, is all of the sudden skipping naps. She's just learned to sit up in her crib, so she'll sit back there for an hour or more and not nap. She gets SO tired, which lets me know for sure it's not that she's ready to give up a nap. She yawns and rubs her face and sucks her thumb, but she sits back there and won't lay down. It got to the point yesterday where she fell ASLEEP sitting up. It was the cutest thing ever - she had the crib bumper flower pattern on her forehead because she had slumped forward onto the bumpers. But, it was also the most frustrating thing because she needed to sleep and wouldn't lay down. Is there anything I can do in this situation? Will it just take her a few days to figure out how to lay back down? Isn't it funny how once you get one thing down to a science, something else pops up? :) Literally for us in this case!
I'm not sure if this matters, either, but I think she may be getting her top front teeth in. I really don't want to start any bad habits as far as going in when I need to leave her alone, so any advice would be fantastic!! I've googled it and can't find anything at all about it, so I'm taking that to mean it's not a long-term problem:)