This is entirely unscientific, but my personal experience:
I'm on Reactine permanently for medical reasons, and the higher my dose, the more stubborn my weight is (I doubled up my dose for two days and "gained" 3 lb, mostly water retention). So, it makes sense that other antihistamines might have the same effects in some people.
The good news is that when I lower my dose again, my weight-loss recovers, so I expect that if your body is reacting like mine to antihistamines, you'll probably be right back to normal once you stop.
Good luck!