Do you have any pics of the underside of the fish, if it was to be bloat it looks likr the early stages, however a pleco becoming 'fat' could have several meanings and causes.
It could be parasites, so do you worm them?
It could be bloat which can be caused by too much proteins, an ammonia spike in the water or internal parasites. So whats the diet?
Or it could be constipated, so have you seen it poo? If not it needs a almost 'laxative' for plecs which is deshelled blanched peas etc etc.
Pure clean water is crucial if any of these cases do exist
Or it could be just fat, but i would rather be safe than sorry.
Hope this helped keep us updated
Good luck!