Ok, so all the facts of a situation are important. Knowing the brakes wore out that fast changes things, and made me google a bit. There is a recall on the front and rear brakes on your bike it would appear. Hit Ma Kaw's website and punch in the VIN and see if it was done.
http://www.kawasakininja300.com/forum/11-ninja-300-issues-troubleshooting/31593-brake-caliper-interference-recall.html
https://www.kawasaki.com/OwnerSupport/SafetyRecall