This is exactly like my DD.
She was sleeping 10-12 hour nights from 4 weeks of age but once we hit the magical 4 month mark, it all went away. She was then only sleeping 2 hours max at a time, and feeding much more often.
It was HARD. Mainly because we were so used to getting some sleep. My DS was one of those babies who woke every 1.5 - 2 hours until he was 12 months old, so he was much worse in reality, but it did not feel like it at the time.
DD is also doing BLW and not getting a great deal down.

This last week, she's had 3 good nights where she's slept 7-8 hours straight. It was brilliant. We didn't do anything to change it, it just happened. She's started getting up on her hands and knees and looks like she's about to take off, and she rolls everywhere... so maybe this is wearing her out a bit.

As others have said, this too shall pass. I like to keep thinking about DS who was horrible up until the age of 1, when something finally clicked and we started getting 14 hours out of him a night!! He is still a fantastic sleeper now at just over 2, with the occasional night here and there where he wakes up from a bad dream etc, but 90% of the time he gets an entire nights rest.

I know DD won't do this forever, and while it feels horrible right now, I know it will improve.
I hope your little one starts sleeping again for you soon. Sleep deprivation is the pits!