Nausea and breastfeeding?!

Almost every time I breastfeed I feel nauseous.

I don’t know why. Is this normal?

I doubt I’m pregnant AGAIN, because I got a nexplanon in my arm as soon as I had my baby. so please don’t suggest I’m pregnant mostly because I just had two babies back to back ( they are 16m apart )

So why do I feel nauseous ??