After taking diet pills, I feel dizzy, my heart beats faster, and I have no strength, but I just want to sleep.

I think it's normal. If you don't take diet pills, this will happen when you are hungry. Did you stop being hungry after taking medicine, and then didn't eat anything? If so, that's normal. If you encounter nausea and vomiting, just eat a piece of sugar, and you will be fine in a moment.