Good morning L.! I feel your pain. I have horrendous allergies also. My son is 9 months old now. When he was 4-5 months old, I was having ridiculously severe allergy attacks daily. When I asked the local pharmacist (who was a friend)about taking meds while breastfeeding & he said most meds are passed into the breastmilk. I had to make that call on whether the allergies were severe enough to take the medicine or to tough it out without the meds. Tylenol is okay because babies take infant Tylenol. What he reminded me was that taking (most OTC medicine) while breastfeeding is a personal choice. You have to do what YOU FEEL IS BEST for you and your little angel.
Another option to think about is contacting your hospital's lactation or nursing consultant. At the hospital I gave birth at, they had a lactation consultant type person on staff. She was a great help to me. I still contact her when I have a question about my son and nursing.
I do hope you feel better. Good luck with your choices. Have a fabulous week!