Warning: I think this is going to be long.

So, suddenly it's like DD is a different baby and I don't know what to do with her.

Background: DD was an amazing sleeper from birth to 4 months. She started STTN very young and for a long time would sleep an 8 hour stretch, then eat, then sleep a few more hours. Then, at 3 months, she started sleeping 11-12 hours straight every night. At 4 months she definitely went through a bit of a sleep regression, as well as teething (although still no teeth), and she started waking 1-3x/night (usually 2x).

She's now 5.5 months. About a week and a half ago we transferred her from the swing to the crib for nighttime sleep. It went well the first few nights (generally she'd sleep through to 5 or 6 am, eat, and then sleep until 7:30). We also started solids and she ended up VERY constipated. So for the past five or six nights it's been hell with three wakeups (I realize that's still not that bad compared to what many experience, but going from STTN to three wakeups is BRUTAL).

I'm so sleep deprived that I had a full-on emotional breakdown yesterday afternoon. I just feel like I have nothing left. My tank is below empty.

I attributed the sleep issues to her tummy troubles. Well, she finally had a BIG poop yesterday, but last night was equally terrible. She woke up at 11:30 (diaper blowout). I changed her and tried to nurse her back to sleep. She wanted none of it - kept pulling off, squirming, crying etc. I finally gave up and just put her down in her crib. She then proceeded to lay awake, silently, until a little after 1. Then she started crying again. I finally went back in and nursed her some more. She didn't fall asleep like she usually does with MOTN feedings, but it at least calmed her down and she feel back to sleep a few minutes after I put her back down. She was then up again at 4 and went back down fairly easily. She woke again at 6:10, we left her (she was just quietly fussing, not crying) and she fell back to sleep until 7:30.

Anyhow, this MOTN behaviour is new. Even when she's woken 2+ times/night, it's always been easy to get her back down. But if she refuses the boob and all other attempts at calming her, I have no idea what to do. I thought if I just left her in her crib she'd eventually fall asleep...but apparently not. She'll just lay there for 1+ hours. We can't even do CIO because she doesn't cry!

Anyone else been through this? Any ideas on how to handle it? Is it possible that it's the 6 month sleep regression (she's 5.5 months, but developmentally 6 months because she was born so late)? Or something developmental - she's really close to sitting fully unassisted?

Here's her Wonder Weeks chart, I guess it could also explain it: