Loach Back lump

[Hakkon norway]

Hi forum, im new here.

My Loach has a big lump where he poops from and seems to be in a lot of pain when we tutch the lump. I was gifted an aqarium with 2 loaches and some other fishes so im pretty new to fishes. Im really worried and hope someone knows whats Going on.

Pictures:
https://imgur.com/gallery/Ur4chnB