It is normal for Rigid Gas Permeable (RGP) contact lenses to reshape the cornea. After you stop wearing them your corneas will relax and your refractive error is very likely to change.
Also, RGPs can delay or mask underlying corneal problems. Being out of them for an extended length of time (one month may not be enough, see
How Long Out Of Contacts For Lasik) will allow them to regress to their natural state and the health of the eye can be re-checked.
The refractive error changes you are experiencing are exactly why you must be out of RGPs before Lasik.