Hello Doctor, Please Kindly Advise As I Am Fainting Anytime

Hello Doctor, Please kindly advise as I am fainting anytime i go up the stairs... When I don't then it means i am taking my time before reaching the top.
I went to the hospital and they did ECG 3 times and also I was made to do an x-ray with blood test and urine test which all came out to be positive with nothing alarming as I was told. Although the doctor told me the first time that I had a tiny bit of blood sample in my urine the first time which they took to a lab for further testing but the end result is still nothing alarming as I was told.
I use to exercise daily and sometimes twice a day and I keep a healthy diet in most cases so I am a bit confused as to what is going on with me and it is kind of scary.

Please what must I do?