What causes my asthma?

I am 17 years old, have had a cough since the age of six. Went to the doctor many times, was never diagnosed until when I was 13 I started wheezing during sports lessons, I couldn't breathe properly. I was given a reliever inhaler when I was 14 and a preventer when I was 15, am still on both. However, my asthma seems to keep getting worse every year - when I was 16 I was put on a combined inhaler as well as a reliever, and now I am finding myself in hospital for no obvious reason, and have had to use nebulisers on more than one occasion. I want to know what could have caused my asthma? I have researched this and although no certain cause for asthma is known (that I have found, anyway), I haven't been exposed to any of the usual environmental factors e.g. no one in my family smokes, and there is no history of asthma or allergies in my family, I am the only one on any sort of breathing medication. Any ideas would be very much welcomed, as I still don't understand why I am getting ill, have never had any sort of allergy test or anything, and it seems strange to me that although I take my medication religously my symptoms still get worse? Any thoughts are absolutely welcome, am reaching the end of my tether here!