Why am I always sick?!?

I'm fourteen years old, I play basketball and I'm in track, I eat pretty healthy, and I take vitamins daily. For some reason, over the past 3 months I've been diagnosed with many different things-strep, an ear infection, a sinus infection, and the flu. I always feel bad, which is completely different from last year when I got sick only once. I'm always achy, tired, congested, and I have a lot of drainage. I'm always really thirsty and my stomach always hurts. Can anyone help me figure out what's wrong with me? Thanks