Bottom of spine feels like clicking/grinding on another bone?

I was in a car accident in January and have since had problems with my back. Only the last month when my back starts aching I sit up straight to help (orders from physio) and it feels like the bottom of my spine in grinding on another bone (it makes a click noise). It doesn't hurt or relieve my back, but I am worried if I leave it it will have bad future effects on my back. Does anyone know what this is?!