Sick Betta- Possibly Constipation?

My Betta has been ill for about a week 2 at the most. He has a large lump on his body and he's lost a bit of colour underneath. Water has been tested and has normal results (pH of 7.4 minimal nitrite etc...) He's in a 15ltr tank with heater (temp 81F) and filter. Because of the location of the tank next to a window, it gets a lot of algae. I've given him anti internal bacteria medication and given him Espom salt baths (20 mins at a time) twice a day for the past 3 days. He's not eaten for a week but he usually eats flake and pellet type food. He can swim fine when he has to but spends the majority of the time lying on the bottom of the tank.

If anyone has any advice or help let me know. I think i've given enough info. If I've missed anything please ask.

Can anyone help me with this plz?

He looks bad it doesn't look like constipation to me. Sorry I can't help much more than that

Really need a betta expert on this one! I've been giving advice on this situation for the past week, only saw the pic a few days ago and I am kinda flummoxed. Based on a description, I thought it had to either be something internal or constipation. Fish has not been fed for days now, the lump hasn't increased but it hasn't decreased either. Looking unlikely to be constipation now after a few days of Epsom baths, these have always worked great for me in the past and I have had a fair few bettas. Can anyone shed any light on what they think is going on?