Please stop feeding fruit altogether - it should never be more than an occasional treat in a small amount.
Guinea pigs have not evolved on eating lots of fruit, and the sugar in them when fed too often can increase the risk of dysbiosis (overgrowth of the wrong kind of gut bacteria, resulting in diarrhea or bloating). They are sugar junkies like humans but it is not doing them much good if you overdo it. The closer you keep the diet to a natural one, the more you actually promote longer health and a longer life span; cutting down on carrots, regular fruit and processed treats/ the amount of pellets you feed can add 1-2 years in my ow experience and take more piggies from the lower end of the average life span to the upper end or beyond.
Plenty of fresh grass can cause soft poos, too. Just leave off any fresh food in the evening and problem should solve itself - the rich fresh spring grass counts more towards the veg portion than the hay portion.
Long Term Balanced General And Special Needs Guinea Pig Diets