I have bad allergies and was told ceterizine or loratadine are OK. It's also fine to use nasal sprays and eye drops. I've been relying more on those as the tablets you can take wear off after a couple of hours for me.
My Dr sent me this link to the breastfeeding network website https://www.breastfeedingnetwork.org.uk/factsheet/antihistamines/
I used to take loratadine - the NHS website says it’s safe to take whilst breastfeeding.