Sounds to me like it could be an anxiety attack. These things tend to have a snowball effect. You start to get anxious for one reason or another (feeling strange from being high, paranoia) which raises your blood pressure, can make you hyperventilate, make you dizzy, etc. This makes you even more anxious so the symptoms get worse and so on and so on.

I used to get them driving. I don't know about where you are but people here in California drive like maniacs. Its bumper to bumper at 75-80 mph. One time I started to think about what would happen if I ever fell asleep at the wheel or just passed out during rush hour. This freaked me out and caused an anxiety attack which got me dizzy, made me feel like I actually was going to pass out, which freaked me out even more. After that first time I'd get them a lot, sometimes I would have to get off the highway and pull off the side of the road and chill for a bit. I started to think I had a brain tumor or something crazy so I went to the doctor and was told it was anxiety attacks.

Anyhow, enough about me. Next time this happens try doing what I do. Just take a couple of nice slow deep breathes and tell yourself "this is just an anxiety attack, just chill and it will pass" . Most likely it will and you can enjoy your high.