Benadryl is considered safe in pregnancy up to 50mg every 8 hours. I get rashes all the time in pregnancy and OB said take benadryl.
As for not taking it unless you can't breath that's too late you need to be at an ER asap call 911 if you can't breath!
Polarmine is an antihistimine that is safe during the first two trimesters of Your pregnancy however not after the sixth month.
I had a similar rash the doctor seemed to think it was heat rash due to the pregnancy
I was told by my OB GP and pediatrician not to take any anti hisamines like benadryl during pregnancy or BF unless I am having trouble breathing, then once or as little as possible as they can dehydrate you and cause contractions and somethign else but I forget, ask your dr.