6 months post op and cold medicine?

I am 6 months out from my rny and haven't been sick since before surgery (yay!****il yesterday that is. 

I have the dreaded summer cold. I haven't taken anything since surgery. Not even Tylenol. I also have hives all over the place at the same time. So I took a benadryl. Only one tablet and it is knocking me on my butt!

So here's my question: does medicine effect you differently after surgery? Do you have any tips to get through a cold without taking meds?

It can hit harder and quicker 

I get hives now at least once a week. I thought it was just stress induced but then they started hitting me when I'm calm too and relaxing. PCP and surgeon believe that an allergy test will be a waste of time and money since antihistamines work for me.

As to effect - the antihistamine works to calm the hives, but it doesn't do much more for me. I had Tylenol once since surgery (non codeine) and it did nothing. My anxiety meds dosage had to be changed - it was less effective after surgery.

Now, I'm supposed to avoid all over the counter meds other than the antihistamine. Doc said that since the pain meds have no effect, try just taking a nap and if pain is ever bad enough to need something more, go to the ER for narcotics. I've been lucky enough that so far that hasn't been necessary though.