Recurring colds

I've had 5 colds since August, and I had a total of 6 last school year. I haven't had any serious illness in that time, just colds. Someone suggested that I may have allergies, but I'm skeptical because this isn't seasonal, it's constant. I'm 16, technically female (although I'm a trans guy), 5'2", and 115lbs. I live in the southwest US, in a very dry, hot environment. I don't have any existing medical conditions. I take birth control and theanine every day.

Any ideas as to what's getting me sick all the time?