I wouldn't worry too much about the lettuce, I always gave it to my piggies in the past before somebody decided it was bad for them and they never had any ill effects. I buy those bags of different lettuces for them now from Tesco and they love it and they are all healthy and happy. My oldest piggy is 6 years old this month.

Maybe take her for a check up to the vets if you are really worried about her.