The main mechanism of vigra action can some time lead to decrease in blood pressure in some people to a level that it can present in the way you are feeling.
Decrease in blood pressure can lead to palpitation
and tremors which you suffer when you have this drug. This drug has also interaction with drugs used to maintain blood pressure and is not advised for some one you is taking some sort of cardio vascular drug.
If you have some sort of cardio problem or if you are taking some drug related with cardiac problem than I would advice you to consult your doctor before using vigra in future or even if you don't have any thing and have problem with vigra than stop taking this.
Thanks and take care.