I hope I can help guide you here.
No, tylenol, which is what acetaminophen is, won't give a rash when combined with either the cipro or the flagyl.
Even though you write that you believe you have had both cipro and flagyl before, I can't rule out the possibility that the hives are due to an allergic reaction to either the cipro OR the flagyl. Especially as you don't seem too sure about it. Allergies to medications can be severe and life threatening, and can worsen quickly. Yes, if you have OTC benadryl, take as directed. BUT, STOP THE FLAGYL AND THE CIPRO...do not take them again until you have discussed this rash you have with your doctor. If you have a digital camera, take a picture of the rash you have now so your doctor can see it; its a good idea to call him or her tomorrow.
Meanwhile tonight: IF you start to have ANY difficulty breathing, or chest tightness, or swelling around your face or lips, CALL 911. That means the allergy, if this is one, is worsening, and it may be a matter of time before you can not breathe AT ALL. It may not even be a bad idea to GO NOW, because even though RIGHT NOW you can breathe and only have hives, allergic reactions can progress that fast. But, you do have benadryl, and that counters the reaction. BUT, if its severe, even the benadryl won't be enough, you will need hospital intervention.
Of note: IF the benadryl is liquid, and IF it has alcohol, not a good mix with the flagyl...alcohol and flagyl do not play nicely within the human body; stomach aches and cramps will be the unhappy result.
I hope I was able to help.