The "grinding" sensation maybe fluid or shifting of your implant against your chest wall. This may simply be residual swelling in your tissues against the implant. Wear a tight compression bra and consult your plastic surgeon. Until then minimize your exercise activity level.
You might have a textured implant that is moving within the pocket, but the answer is no, you should not have a grinding feeling behind your implant. We suggest a snug bra and a visit with your surgeon.