Heya
It sounds like it's possible she is mainly getting foremilk. Do you change breast every 40 minutes? Foremilk digests the most quickly because it has less fat content so baby won't be full for very long at all. It can also cause them to go a lot and have green poos because of the high sugar content but they will still gain weight normally, even extra weight gain.
We were having this problem because I stopped putting her back to the same breast and started offering the other too soon.
I found this resource https://www.llli.org/faq/oversupply.html
It says about 'block feeding' which is what helped us. Basically within a certain period, I was doing it for at least two hours, offer the same breast rather than the next breast, this means they will access the fatty hindmilk and go longer between feeds and poo a bit less (in our experience).
I warn you that at first my little girl did fight drinking from the same breast a bit because she was so used to the thin foremilk coming out almost instantly but within a day she was used to working a bit harder to get the fatt milk out and was happy and everything is fine again.
Now she tends to do a full feed from one breast and if she wants a snack before her next full feed I offer her the same breast. Then a couple of hours later she can have the other one.
Hope this helps and you can sort it out
xx