The hints in the previous posts are all good. I try not to cut in the same spot on my mat. That helps make the mat last longer. I don't use the lines on the mat to measure so it is easy to move around. The mat I have now is really old so I just turned it over to extend its life. Works just fine. Try cutting in a different area of the mat and see if you have the same problem.
I like Olfa mats as they have lasted the longest for me and have not warped like the Fiskar mat I tried.