Do i calculate my next menstrual cycle from the last day i bleed from plan b ? or from my original menstrual that i had a week before that?

Depends: It depends on where you were at during your cycle when you took the plan b, whether or not you had a withdrawal bleed from the Plan B or not. You likely have to wait a cycle to see when you ovulate and when you menstruate next. In the meantime, definitely use a reliable form of birth control as you may not be able to determine when an egg is going to be released.