Is this a regular baby thing or should I be concerned???

My babygirl was born 5 weeks prematurely at 5lbs 5oz and was in the NICU for 54 days because they couldn't find a formula that worked well with her and couldn't find a nipple she could latch to.

At first they had her on Neosure through her feeding tube and whenever she started trying to take bottles they switched her to alimentum. After a couple weeks she wasn't able to finish her bottles they did a test and found out she has acid reflux and the way her tongue is built makes it difficult for her to swallow. That being said they switched her to enfamil AR and started her on the MAM pacifier and nipple and she started doing amazing.

We got out the hospital July 18th she weighed 7lbs 12oz and was taking 4oz bottles every 3 hours. On July 21st she weighed 8lbs still taking 4oz every 3 hours. Now September 2nd she weighs 9lbs 9oz taking 6oz every 3 hours.

I don't think that's right. I feel like she's not gaining weight how he should be. I'm not sure if it's the formula or if I'm feeding her too much but she's always spitting up/throwing up after every bottle. Her stomach is constantly hurting her. Pooping constantly hurts her. Passing gas constantly hurts her. I feel like she gets hungry again wayy to quick for a 3 month old. I've taken her to her pediatrician multiple times and she keeps saying it's normal and that she'll get used to it.

Any info or advice would help!!! I'm 19 and she's my first baby, I have no clue what I'm doing some days and I have no clue what's normal and what's not.

thank u if you've read this far