They need a variety of veg every single day, including a vit c rich veg. Every day my piggies get lettuce, cucumber, a slice of bell pepper and a herb (usually coriander daily).
If she has only been eating lettuce and that causes her to have soft poops, then I would cut the lettuce out entirely, try her on herbs and see what happens.
Guinea pigs need to be weighed every week as part of life long care. It’s the only way to know they are eating enough hay. You then switch to daily weighing when there is a health concern.
But, if she is having a hard tummy and diarrhoea, then it sounds as if there is a digestive issue. You need to be absolutely certain there isn’t a health issue, I would get her checked by a vet again if this is continuing.
Long Term Balanced General And Special Needs Guinea Pig Diets
Edible And Forbidden Veg And Fruit List With Vitamin C Grading
Weight - Monitoring and Management
Digestive Disorders: Diarrhea - Bloat - GI Stasis (No Gut Movement) And Not Eating