Help! Regression to newborn pattern :-(

[OpheliasWeepingWillow]

My 10wk old dd has switched to a newborn night feeding pattern after I had to abruptly switch to formula last week for medical reasons. She's up every hour and a half to eat just an once or so... Same as in the day.

Why and what can I do?

I don't claim to be an expert so take me with a pinch of salt, but in my experience, changing to formula or changing brands of formula takes a lot of getting used to, for some babies more than others, so it could just be an adjustment period, where she is getting used to what amount she needs to drink to be full up, whereas before her body was used to taking X amount, getting sleepy, waiting for next feed, now she takes that amount, gets sleep y but wasn't full yet. If that makes sense.
Alternativly it can just be a developmental thing and nothing to do with the switch to formula at all, sometimes there will be a week or two of cluster feeding and then normal service resumes till next time.
There are plenty of similar theories and explanations it could be and while I know time moves horribly slowly when you've got tiny babies I'd give it another few days and see if she adjusts on her own and then go and have a chat with your health visitor. Cluster feeding is very common a I said for many possible reasons but in some cases it can be indicative of something else that needs looking into, for example reflux.
I hope my essay post gave you some help and I'm sure people with more experience can give you much more information shortly.