4 month old & breastfeeding

It seems as if my milk supply has slowed down. My breast really only get engorged maybe once a day. Through the day I don't feel let downs or anything. I leak often but also when baby is feedin, my breast feel really emptied and she could really nurse for hours, eventually she falls asleep. But she never lets go off my chest and I think it's because I'm not producing too much milk. The first two months I was producing a lot of milk and slowly I've noticed my breast always feel emptied and it's now worrying me. I also give my Daughter formula because I feel like my breastmilk just isn't enough. What can I do to increase my milk supply? I just recently went back to work. I never really pump, unless I feel really engorged. And now returning back to work, we just feed Our daughter formula.