Breast feeding help

Makayla
My milk supply is now just about gone and i dont understand why because i was exclusively breastfeeding and pumping. I didnt start introducing formula until my baby seemed to not be getting enough. I had to take him to the doctor 2 days ago because he had stopped going poop and was in a lot of pain. His pediatrician said it was because of the formula, but i have tried 3 different kinds and his little tummy just cant handle it. Im in desporate need of tips or product recommendations to help get my supply back up. Thank you.