I'm sorry you haven't been feeling well and I hope you're better soon.
You mention that nursing is going really well for you. From your message I'm assuming that you would rather continue nursing and not wean your daughter. Is this a correct assumption? If so, then I strongly urge you not to wean. It's simply not necessary in this instance. If you really would rather wean and are ready to be done with nursing then that's another issue. However, don't make this the reason/excuse for doing so, since it's just not a valid reason.
That said, if you do want to continue nursing you have many options. First, check the meds out on www.kellymom.com. It's a great website and you'll learn a lot about which meds are safe. I personally would NOT trust your allergist to know which meds are safe while nursing. I had to see a # of specialists while I was breastfeeding and they all wanted to place me on various meds. Most admitted they didn't really know how safe the meds were, but figured they were ok. And, just because a med is safe to use in pregnancy doesn't mean it's safe to use while nursing. All medication passes through to the breastmilk in some form. Most however is in miniscule, trace amounts that are believed to be perfectly safe. Others are strict no nos. The only people I trusted to tell me if a med was safe while nursing were my son's pediatrician and even moreso, our pharmacist. I was prescribed a drug by 2 drs who said it was fine. I asked our pharmacist who looked it up and said, "NO WAY." So, I opted not to take the drug.
And while I'm sure you're miserable, the condition you are treating is not life threatening and you could agree to forego the meds. The dr was actually quite aggressive on the meds as a starting point and there's no reason you needed to go whole hog right out of the gate. There are many allergy drugs you could try if one of these don't work. And, there are also many homeopathic (netti pots) remedies that have been shown to work just as well.
If it were me I'd check with the pedi and pharmacist to see if they agree on the saftey of these meds while nursing. If they do, then you're fine. If not, I'm sure there are other meds that are safe. Or, maybe just suffer through a few weeks and see if you get better?
Good luck.