yes a reaction can affect you. when I have a reaction the next day I am always exhausted

It does the opposite for me. It makes my heart race

I always have what I call an antihistamine hangover, light-headed and groggy.

It absolutely can! The first time I went into anaphylactic shock, I didn't walk for 2 weeks afterward.

Yes. Exhausted and something like a hangover. You body was stressed and it needs to rest and recuperate. Who needs amusement park when you have bad reactions, you get all the ups and downs you can handle!