What happened?!

A few days ago, my little girl suddenly started having a ton of trouble falling asleep. She's never been one to fall asleep right away, but play in her bed a little and then go to sleep on her own. This week though, she has been absolutely screaming crying. I feed her, make sure she's not in pain(not sure if she's teething or not), check her diaper, everything. She's fine until I leave the room. Then she cries, sometimes for 20-30 minutes, until she slowly falls asleep.

I can't go in and comfort her without the crying getting so much worse the instant I leave, and I'm not about to sit there and wait for her to sleep, I'm not starting that habit.

is this just separation anxiety? is there something I'm missing? this is so extremely and suddenly different than her usual behavior.

please help