Hamster lost weight but still energetic and eating

[ChunkyPineapple]

Tl;dr - dwarf hamster still eating well and acting normal but gone from 63g to 42g over 4 months. Not sure whether it's an age related thing or requires vets. Other hamster in same room had pests in November after arriving and received treatment. This hamster saw vets for abscess in November but I think there's another lump on his stomach now.

More detailed:

I have a dwarf hamster that I got in March 2021 when he was young, so I guess he's about a year old. I noticed recently that he has lost weight and looked smaller than before and just weighed him for the first time since october (now highly regretting that). He has gone from 63g to 42g. He has always been fairly chunky and about 60g since I started weighing him in April 2021. His fur on the back of his neck is also thinner but not bald. He still loves food - I give him a mixture of select science nuggets and the pets at home muesli and occasionally gets mealworms, millet sprays and I often sprinkle some foraging mixtures around his cage for enrichment. His activities have remained the same - he's still running a lot on his wheel and wanting to come out.

Points that may be relevant:
In November, I took him to the vets as he had a lump on his stomach which ended up disappearing just before the appointment and the vet said it looked like he'd had an abscess but it'd gone away on its own and it didn't return so he didn't have any treatment for that. However, I'm not sure if he had another one then too but there still appears to be some form of lump (he wriggles around a lot so it's hard to check)
Also in November (after vets happened), I adopted another hamster. He seemed to come from a poor condition as after a few days when I was cleaning out his cage, I noticed it was infested with fleas or mites. He has been treated and I haven't seen any evidence of life in his cage since then and I haven't noticed any in the hamster this post's about's cage either. However, they are both in my bedroom (opposite sides to minimise stress) so I thought it's still a possibility. I can probably attach a video of what I saw in the adopted hamster's cage as they were visible and jumping.
He does itch a bit but I'm not sure whether the frequency has changed.

I'm just not sure if this is a seasonal/age related thing or whether it's a genuine health thing. I am very prepared to take him to the vets and get treatment and open to any suggestions about what could cause this.

[Ria P]

The lump on his abdomen, is it near his scent gland?

I take it that you've checked his teeth?

He may be hoarding most of the food instead of eating it. Personally, i'd buy a better quality food than petsathome and give a daily pinch of brown linseeds for his fur. Weight loss like that at only a year old is concerning. I'd check his urine with a diastick to see if anything shows up or get him checked by a hamster knowledgable vet.

Sorry i can't be more helpful but weight loss can have so many causes.