This is not why you didn't feel the Adderall. I have taken antihistamines, even the one you mentioned, in combination with amphetamine based drugs and the "high" was never compromised. Benzodiazepines are a totally different story though as I am finding out.
As I have mentioned, you might have gotten a shitty generic brand. Some of the generics for Adderall are absolutely horrible, and do nothing more for me than raise my blood pressure and heart beat.