Illness/Sickness: New axolotls with swollen lumpy tummys

Hi wondering if anyone could help me, im new to having axolotls and worried there not well:uhoh::uhoh: Ive had floss and flump now for two and a half weeks and there lovely, happy and friendly. but ive noticed today that both there tummys are swollen and lumpy. I have fine sand in there tank with no rocks or anything that they could swollow, but they were kept in a tank with gravel at the pet shop . Since we got them there tummys have looked very dark like they have gravel inside? (they are white with pink gills so quite see through:happy They have been eating happily. We feed them once a day with either wax worms or blood worms or earth worms, which they seem to really enjoy and happily eat. The day we got them i removed one poo from there tank, at least i think thats what it was, it looked like a small brown pellet? But nothing since so could they be constipated? Could they be blocked up with gravel? And if so what can we do to help? Hate to think that there in pain

Pics would greatly help here. If they're eating that's a good sign, there may not be an issue.