My 3 month old is not having a lot of wet nappies as she used to. should i be worried? she's on breast milk exclusively?

Normal: Breastfeeding babies frequently utilize most of what they take in and therefore would not have as many BM's/pee diapers since there is less waste products. As a rule of thumb, a 3 month old should have about 4-5 wet diapers. In addition, by 3 months, the baby's system is getting much more efficient in both feeding and elimination. Talk to your Dr at her 4m visit.