Help!!! 2yr old sudden fear of sleeping alone

So for over a year now my daughter has been a great sleeper. Never fought us and sleeps a good amount of time. Recently though, it's like it all changed. She screams and clings to us at bed time. And it's not an "I just wanna act up because I don't want to be in bed" kinda thing, it's more of, "I'm deathly afraid to be alone" kinda thing. The doctor wanted me to let her scream herself to sleep, I tried twice and it lasted over an hour before I took her in my bed to sleep. From there she slept great. But if she notices I get up or anything she freaks out again. what the heck is going on? she obviously can't sleep with me forever and I don't want her to feel this upset about sleeping because it's not a punishment. did anyone else's 2 year old go through this? I'm starting to really worry.