Trouble sleeping out of nowhere?

Amy
Baby M had been sleeping through the night consistently since 6 weeks old, but suddenly sleeping is terrible! She screams in her sleep, wakes up 2-3x per night, won't nap during the (but will scream and scream)...
She's been falling asleep on her own for over a month now, so sleep training isn't the issue either. 
Anyone else going through this sudden change or now what new developments she may be going through that could cause it?