The only options you have, as a breastfeeding mom, are the progestin
only options. Such as the mini pill, Mirena or Paraguard IUD, depo shot. I'm not familiar with nexplanon so I don't know if that has progestin only or if it's combination.
Keep in mind that after having been pregnant and going through the trauma of childbirth, plus currently breastfeeding, your hormones are all over the place and it's going to take months to calm everything down to normal levels again.