nausea when i don’t eat

i’m not pregnant , i’m on nexplanon !! but i noticed when i was pregnant when i didn’t eat i’d be nauseas i had my son june 10, 2019 and i had no appetite while i was breastfeed but i wouldn’t get nauseas when i didn’t eat !! when i stopped breastfeeding that’s when it started and i stopped breastfeeding november 2019 because my supply tanked !! ever since then my appetite came back and now if i don’t eat i feel like complete shit , when i eat i feel a lot better but idk the reasoning behind that , any ideas ???