Hi and welcome to the forums. An adult pig should only get 1 cup of vegetables a day with fruits being an occasional treat (a couple times a week). To much sugar can cause many health problems for them. If your guinea pig is younger 1 cup may be to much. Here is a list of veggies you can feed and it is color coded to how often they should be fed
https://www.guineapigcages.com/foru...-charts-poisonous-plants-list.html#post254730 .
What kind of hay and pellets are feeding? Most store brands of hay and pellets are junk. What we usually recommend are Oxbow pellets and hay (available online and in some pet stores), KMS only available on line (broken link removed) , there is another brand I think called Sweet Meadow (?) which I have never used .
Does your pig have access to unlimited hay? And how much pellets are you feeding? The recommended pellet amount is 1/4-1/8 cup per pig.
How old is your pig?