Increase milk help!

I was doing well I am 4 weeks post, I was supplying a whole pitcher full throw out the days and then able to also make her feedings, so when she came home we was on 8 bottles I would pre make my bottles & fill my pitcher for next day feeding & whatever was left I would store. However I missed a whole day or pumpin n since my supply has been super low I use to be able pump almost 4 oz each boobs, 15 min pumps. Now sometimes I have to do 20 & barely make 2oz I know my girl is going to require more soon I exclusively pump. ANY ADVICE?! this also is my first breast-feeding journey, and I would like to continue with.