Alrighty, there are other threads on this but I can't remember where they are.
Check out podcast 22 HERE. She goes through many of the usual drugs what effect they have on baby if still breastfeeding.
The main thing is whether you're having normal, ovulatory cycles or not while breastfeeding. If you are, then you're fertile regardless. If you're not, then it may be impacting fertility.
I continued to breastfeed through a HRT FET cycle, though the drugs killed my supply anyway (which is the other problem with elevated estrogen levels).