I keep getting sick!?

I have been sick every 3-4 weeks for the past year almost. Sinus infections, bronchitis, tonsils swollen to the point where my throat is just about closed (they didn't know what it was), stomache problems (pains and indigestion)...it's just endless. My doctor has taken blood numerous times, throat cultures, and all kinds of stuff.... I just have no idea why I keep getting sick! I am not overweight or unhealthy. Im a 21 year old woman! There has to be a reason. If anyone has any ideas please let me know. Hopefully there are some doctors reading this :o)