I agree with the others re calcium pee.
Excess calcium is excreted in the urine which is normal if it happens occasionally. It will feel chalky. If It happens Once or twice is ok but if it happens a lot it means something is wrong in the diet.
If it feels gritty then there is a problem as crystals are forming.
The fact you are feeding unlimited of the young piggy pellets is most likely where the issue is.
Piggies should never have unlimited pellets of any type but particularly not of the young piggy pellets. This is because the young piggy pellets contain alfalfa. Alfalfa is a legume (so not a grass) which guinea pigs should not eat - It contains higher calcium levels.
The young piggy pellets are a marketing gimmick and are entirely unnecessary to ever be given - piggies can eat the normal adult pellets from birth. The amount of additional nutrition youngsters need is actually rather tiny.
Alfalfa can be given supplementary to newborn pups under three weeks of age but after that it isn’t needed.
If anybody is choosing to give the young piggy pellets then it is strictly one tablespoon per day due to the higher nutrient levels (piggies under four months can have two tablespoons of normal adult pellets instead to give them a little extra nutrition) they most definitely cannot be fed unlimited, and you need to stop feeding young piggy pellets entirely by the time they are four months old. Now yours are at that age, please do switch to one tablespoon of normal adult pellets per pig per day.
Pellets and water bring most calcium into the diet so it is there which care needs to be taken first.
Section 6 of this guide explains
Long Term Balanced General And Special Needs Guinea Pig Diets