Nighttime nightmare

I’m in desperate need of advice. My 11.5 month old had been waking up 2-3 times per night now, sometimes awake for 2 hour stretches. She will fall asleep in my arms then when I go put her down in her crib she wakes up and starts screaming. This has been going on for the last 5 nights now, I’m running on 3-4 hours of sleep and having to care for her and my 3 year old all day by myself. I’m at my breaking point and need some advice.

Is this a sleep regression? I’ve tried letting her cry but she just keeps going, sometimes crying for an hour.