Why do I suddenly started breathing fast and my heartbeat was rapidly beating and I was feeling it from heart as well as from both ears after riding a cycle after so long time, I was about to faint and I was unable to see clear for a while, I was unable to stand because I started shivering very badly and lost energy and sat down for few minutes I was completely blind, I often feel problem in breathing but when I visit doctor he says everything is normal. He says you are anxious about something, yes I do agree that I am mentally little bit weak nowadays is it a reason for that or something else? Please tell me I haven't told it to anyone yet in my house is it serious?