Stop supplementing now ebf

So my baby was drinking breast milk along with formula since she was born she’s now 4 months . She got sick for a few days and during that time I only wanted her to have breast milk and we stopped supplementing and now she isn’t peeing as much and she might’ve lost a little weight . We spoke with her ped over this concern and they suggested I pump the milk so I know exactly how much she’s eating and go from then on . Has this happened to any other moms and babies and how did you overcome it ? I feel like absolute crap for not weaning her right .