Thanks for posting,
It seems your baby is having a neonatal colic
which could be normal if it lasts for a couple of hours. But if the stool becomes frequent with mucus then it is surely something you should definitely be worried about. Your baby might be having a gastroenteritis
(which is very likely given that you were breast feeding her and i assume you were keeping all hygiene). Just keep breast feeding her and no more worries, as it is likely that your baby is having a neonatal colic that will go away on its own.
Please if the baby continuous having the colic, do visit a pediatrician
as soon as possible.
Wish you the best