Low milk supply 😔

I have strictly breastfed since my son was born. He is three months old now and has had only a few formula bottles. The past few days I have had to give him at least two formula bottles a day because my milk supply has gotten so low. I don't know what is wrong. It makes me so sad to think my body isn't producing enough when I used to produce a whole bunch of milk. Any tips to increase supply? Please help!