Hi there. I have a 9 month old and remember going through this all too well. The main thing to keep in your mind is that this is only temporary, her sleep will settle in time, you've just got to make it through this period. My little girl slept perfectly until she was about 10 weeks old then was very much like this.
Firstly, is she getting enough sleep in general throughout the day? When my little girl got overtired at this age she would settle to sleep easily enough most of the time, but would then wake up in less than an hour and keeping her asleep was a real struggle. I know every baby is different but you can get a general idea of how much sleep they should be getting online or from the health visitor. I wouldn't worry about the formula because exclusive breastmilk is so good for babies and she will still need night feeds because shes so small. Other reasons we've had our little girl wake were teething (this can be the teeth moving around, not just starting to erupt, and my little girl struggled with it from time to time from about 3 months) or just not being able to see you. We got a night light that spins and projects stars on the ceiling and walls which our little girl enjoys watching and it lights up the room enough for her to see us. There are loads you can get for about £10 on amazon so that's always an option.
Obviously, figuring out what's waking her would be the easiest solution but I know how difficult that can be. When my little girl was like this, the most effective methods we found to keep her asleep were swaddling her in a nice blanket and co-sleeping, which pre baby we said we'd never do but I've found it's the best way to keep my little girl asleep sometimes, particularly when shes teething or post jab, and as far as I am concerned all of us getting a decent nights sleep is more important than where she 'should' be sleeping. I get criticism from my mum for it because she still sleeps with us from time to time but I would much rather a well rested baby (and mummy as a result), and theres something quite lovely about waking up to your little one smiling and murmuring at you. My partner and I have her between us to stop her falling out the bed and when hes working nights I just put pillows on the other side of her to keep her safe. It's really worked for us so I would strongly recommend it, plus shes close by to feed which is always a bonus. I know other people use the cry it out method but personally I hate the idea as your baby only stops and falls asleep after exhausting themself and giving up hope of a parent coming to help, but that's just me.
Also, do you have a partner or family member able to come and help out for a few days? Just someone doing odd bits around the house helps sp much, or if you have a partner you could pump some milk during the day and let them take every other night feed to give you a bit of rest. Try not to get too caught up in finding out what's causing your little girl to wake just yet, I'd focus on improving her sleep first and getting some rest yourself.
Remember, it wont last forever and you can get through this. It may feel like a nightmare at the moment but it'll pass and you'll probably forget her sleep was ever this bad.
Good luck! X