Seroquel makes me very sleepy and while it doesn't help much with anxiety (it used to, though) it hasn't produced what you're talking about. Could you be having a panic attack? Have you been anxious about increasing the dose? That said, I think it would be perfectly understandable to get super anxious about the fast heart rate in and of itself, I certainly would!

I'd call your pdoc and let him/her know about this possible side effect. Maybe he'll lower the dose again or choose a different AP that doesn't make you feel that way. Best of luck to you and I hope you feel better soon!!