Kellymom.com says :
It's recommended that any estrogen-containing contraceptive be avoided until baby is at least six months old AND after baby is well-established on solid foods.
Combination contraceptives contain both progesterone and estrogen and come in several different forms:
The combination birth control pill (Alesse, Yasmin, Seasonale, Mircette, Loestrin, Lo/ovral, Demulen, Desogen, Nordette, Ortho Tri-Cyclen, Triphasil, Norinyl, Ortho-Novum, Ovral, etc.)
the monthly injection (Lunelle)
the birth control patch (Ortho Evra)
the vaginal ring (NuvaRing).
Milk supply: Estrogen-containing contraceptives have been linked to low milk supply and a shorter duration of breastfeeding even when started when baby is older, after milk supply is well established. Not all mothers who take contraceptives containing estrogen will experience a low milk supply, but these unaffected mothers appear to be a very small minority.
Safety: Both progestin (progesterone) and estrogen are approved by the American Academy of Pediatrics (AAP) for use in breastfeeding mothers.
I don't think you should worry about your baby 'absorbing' stuff from your pill as much as how it might affect your lactation. It would make sense for your supply to be now improved. I don't know much about the IUD but maybe it's worth looking into. I'm using condoms (TMI!?)