I took zyrtec at 5 pm and benadryl (diphenhydramine) 9 pm is that ok?

Yes: If you take the recommended dose, it should be okay to combine these two medications.

Yes but why?: While there shouldn't be an issue with these 2 antihistamines, it would be helpful to know what the underlying trigger is in an effort to avoid needing benadryl (diphenhydramine). Benadryl (diphenhydramine) is associated with more side effects and potentially care over effects the following morning in some.