First Law of Thermodynamics — Conservation of Energy

The energy was used up for your movement. Regardless of surface, no movement can last forever.

It became chemical energy used to build muscles inside your body.

It became heat due to the friction of the grass you were rolling on slowing you down. If you were on a perfectly smooth surface, you never would've stopped.

It became kinetic energy of the hill as you made it move backwards (very slowly, since you are small compared to a hill).