Increased Heart Rate, Cramps, Breathing Difficulty, Constant Headache. Advised Ct Scan And MRI. Suggestions?

I am 19 and have been having heart problems. I ve been in the ER several times when my heart rate is over 140 and I would get very dizzy and black out. I had an EKG done that showed abnormalities as well as blood drawn from an artery in my wrist that showed my gas oxygen or something was low. I have a doctor here on campus that I have been keeping up to date, and he had me wear a holter monitor for 24 hours and it showed that it was abnormal and that my heart wiring was off or something. When I get in a lot of pain it feels as though my heart is cramping and makes me lose my breath until it stops. I have also been having a constant headache for quiet awhile in the back of my head as well as weight lose, and I tend to not see people in my perrifial vision anymore, which is annoying. I am 5 2 and weight 115 pounds so I am not over weight and I work out regularly, but I have lost 8 pounds in two weeks. My doctor was talking about getting a CT scan as well or an MRI of my brain which I don t understand why. Anyways, In the meantime I just want answers cause right now I am not getting any and it is honestly getting worse and worse every day. I do plan on seeing a cardiologist as well. But would like to have an idea of what is going on. Thank you so much for your time.