I think it may be the Mach3 configuration settings for the X axis. Under the configurations tab select Homing/Limits and try selecting Reversed for the X axis and you will probably have to select Home Negative for the X axis also.
Try it out after making the changes and be ready to stop it if it does not home on the switch. I think my X axis was reversed also. I checked both axis to make sure they are + and - in the correct directions. I hope this helps solve the problem but the Artsoft Mach3 website has some good videos for troubleshooting and other things so you may want to download them and check them out.