Nursing and pumping

I exclusively breast feed my 3 month old daughter but I’ve noticed she is still starving after a feed so I’ve been supplementing with formula. Is it bad to supplement after every feed? How do you supplement with formula? I’m sad because when she was a newborn I always had more than enough milk and I always still had a ton left to pump so I could build up my freezer stash. Well now my little one has gone through the entire freezer stash and I never get a chance to pump because when I do, that leaves my little one with not enough to eat when nursing. Any advice would be awesome.