I was really suffering last year when I was pg, and asked my obstetrician if I could take anything. He said piriton is OK when pg - he double checked for me in fact. Apparently he prescribes it more for other allergies than hayfever. He advised me to take it only if I really needed it. The flip side is that it makes me really drowsy. But it may be worth asking if you can take it when bf.
Btw, I have found that putting vaseline or lip salve around my nostrils works just as well as more expensive stuff marketed as a hayfever barrier.