Pepto-Bismol - took 2 doses last night and woke up with itchy hives. Is this an allergic reaction?

I think you should go to the doctor - no offense Zoo - because you already have hives, and yes, Benadryl will probably be great - but what if not?
I've taken Benadryl for prickly heat and it worked, but, really, even tho it is OTC, it still is a drug.

Dzoobaby is probably completely right, but also think about any other thing you may have taken. Some drugs don't mix well together & can cause a reaction also.

Yes. That is an allergic reaction. If you are not having any trouble with your face, lips or tongue swelling and no difficulty with your breathing, you can take some Benadryl for the itching and discontinue using the Pepto-Bismol and never use it again. If you use a medication you have had a previous reaction to, you may get a worse reaction the next time you take it. If you notice your face, lips or tongue is swelling or you have difficulty breathing you need to be seen in the ER immediately. Are you allergic to aspirin? Pepto-Bismol has a salicylate ingredient that is similar to aspirin. You may want to avoid any products with salicylates in the future.

https://www.drugs.com/mtm/pepto-bismol.html