I was wondering why space-time is foaming or bumpy at the microscopic level or plank scale ?

"Foam" is a euphemism for small-scale fluctuations that occur, because the Heisenberg Uncertainty Principle dictates that we can't know the energy exactly; we need to look longer to get a better idea of what the energy actually is. So on short time scales, when the energy uncertainty is large, you can actually have particle and antiparticle pairs existing.