The short answer is: I don't really know.
The longer answer is everyone is different, our bodies are constantly changing, and things like illness, diet, sleep, stress, etc. are also factors. So, something that may affect one person may not affect another person. Or, it could affect a person one day and not affect the same person at another time.
Over and over, I read comments like "I did 'X' and then 'Y' happened." That's correlation and not necessarily causation. People often omit numerous other factors that might have affected the situation. Things could have been set into motion days or weeks prior that caused/influenced a certain outcome. But, such attention to detail is often absent. I understand why - because tracking it is neither easy nor fun.
Anyway, again, it's hard to know why you took an antihistamine two years ago and had no reaction (at least, as far as you can recall) and were seemingly affected this time.
I might suggest looking for non-pharmaceutical antihistamines, if that's an option for you. The one I've heard mentioned the most is Vitamin C. However, Vitamin C is supposed to be taken to "saturation" (also called "bowel tolerance"), if it's to be effective as an antihistamine.