What do i do if i was flossing, and the back part of my tooth fell out. it is very small, but it wasn't calculus or plaque. i feel no pain but am slightly freaking out?

No need to freak out. It could be part of a filling, or part of your tooth. There may be some decay in that area. It's great that you don't have pain but that doesn't mean you don't have to have it looked at and have it fixed. Call your dentist for an appointment.

You may have a chipped tooth from a previous trauma or from chewing things like ice or hard candy. Before it becomes symptomatic, see your Dentist. Don't wait.

Since there is no pain symptom, it probably is a small composite filling fell out. If if felt sharp or discomfort to your tonge, you may go to your dentist to smooth it out. If the area catches food (food impaction), a new filling is recommended.