I'm really sensitive to meds, I never took more than 50 mgs. of seroquel at a time. It just made me really mellow, but if you're experiencing tachycardia, then maybe it's not for you.
Sometimes though, you have to give your body time to adjust to the new med. When I went off seroquel, and went on triliptal, I felt strange for the first couple of weeks. I felt like I could even smell the drug coming out of my pores. These are very potent meds we are taking, and it makes sense that our bodies take time to adjust to them. Call your doctor, and ask what he/she thinks, and good luck to you!