I used to take antihistamines for my allergies almost daily. All of a sudden, I became allergic to them and pretty much any cold medicine causes my heart to race and I feel horrible.
I started using sinus cleansing with normal saline and taking more probiotics and generally boosting my immune system and my allergies disappeared. I've had little trouble with them in years.
I think in some way the constant use of antihistamines and decongestants were aggravating my allergies. When I went natural, my symptoms disappeared.