Illness/Sickness: Axolotl seems down, regurgitating food and has bump?

Hello all, I got a young axolotl from my local pet store about a week ago and I'm a bit concerned. He has had a small lump on the left side of his tummy since I got him, and the pet store who sold him had him on small gravel. I was relatively unconcerned, however, because he's been eating like a pig and pooping normally until yesterday. Normally he is fed three small slices of earthworm twice a day. Yesterday, he was only fed once, though he was fed four earthworm chunks, and seemed to have regurgitated one of the pieces. Today he was fed three pieces and either regurgitated all three or never ate one and regurgitated the other two. He's also a feisty little fellow and likes to chase around the chopstick we use to feed him with, as well as any hand that's in the tank, and loves to swim around and chase his ghost shrimp. Today that's not the case and he moves away from anything that goes near him, and isn't swimming around much. I noticed his shrimp near him earlier today and he showed no signs of wanting to chase or try to eat it. I did an 80% water change and the current tank temperature is about 67F/a little under 20C. Any advice is appreciated. Attached are pictures of his lump (from above, picture taken about two days ago) and a profile picture from about 10 minutes ago where his stomach looks empty and no stones are visible.

What are your ammonia/nitrites/nitrates/PH readings? Perhaps one of those is off leading to a lack of appetite?

It could be an impaction, but at this point, supportive care is all you can do. I hope your little guy pulls through!