Almost 4 months old baby not drinking usual amount of milk

[Pieces13]

My baby will be 4 months (IA) in two days. I've been struggling with her feeding pattern for the past two months.
She wasn't born chunky or anything but she started gaining weight with time, used to give her both formula and expressed breast milk as I wasn't producing enough. Once I started having good amount of milk I stopped formula. She hasn't had formula for a while now. Everything was fine until she started struggling and refusing bottles. Had to take her to A&E as on that day she barely had any milk. Found out she had Oral thrush infection. Was given medication and her symptoms resolved in a few days until she started showing same symptoms. Took her to A&E again and the doctor said her infection still hasn't resolved. They gave her more medication and now the course is finished and she's again refusing milk.
There are days when she drinks so well 800 to 900ml of expressed milk and then the amount starts decreasing day by day.
I'm mentally exhausted as this is my first baby and I feel like whatever I'm doing is not helping at all. Maybe it's my milk, maybe I should stop giving her my milk and move to formula. If she didn't drink so well on some days I would just accept that she is not other babies and doesn't need as much amount of milk as babies in her age and weight but I've seen how well she drinks. Even on days when she's not drinking well, I notice how she's drinking and will stop and start crying all of a sudden and then won't drink again.

• Have changed teats moved to Medium Flow teat which is No 2 I think
• Have changed bottles
• Burp her in between feeds in case there's any trapped air
• Always feed her in a calm and peaceful room with no distractions

Is she still gaining weight and making a healthy amount of wet nappies? If not then keep going to dr’s and speak to HV. Could be reflux- try making sure she’s upright/look up feeding positions that help with that. Persevere feeding wise with whatever routine you currently have as if you change too much too often then you can’t start to eliminate issues. Only change one thing at a time and do it for long enough to rule out other things. Bit of a hard slog I’m afraid. Hopefully people like HV will help and have other suggestions. Only 2months til weaning - that might also help even if it just makes any issues more obvious and rules out other things. Could be tongue tie also? Fingers crossed for you finding out a way forward

Thank you so much for your input.
Yes she's gaining weight but not as much as she did when she was a newborn. She was gaining more than 200g per week but ever since she got the thrush infection it has reduced. The two times we had to take her to A&E was because she only produced half amount of her usual wet nappies other than that it's fine. Dr ruled out Reflux, silent Reflux and Cow's milk protein allergy.
I'm already mindful of feeding position to avoid any spit ups. Yes you're definitely right about following the same pattern and making only smaller or one changes at a time so it's easier to rule out the issue.
I'm thinking of contacting my HV and have a detailed conversation about all of this because the GP was no help. A clinician did an assessment after my request and seemed annoyed that I contacted GP rather than HV. Said baby's fine (because baby chose that moment to smile) gave a very generic answer "if her overall behaviour, development and milestones are getting achieved than we don't think of this as an issue"