Demoted and Pay Docked for Accident I Reported Without Notification of New Policy

My question involves labor and employment law for the state of: Pennsylvania.

I know PA is an at-will state. I was driving a tow motor at my company and bumped into a conveyor belt - no damage done but no one saw me do this either. I felt the right thing to do was to report it. They sent me home from work to take a drug test, and I couldn't come back until cleared from the drug test. Of course, it was negative, so I went back to work yesterday. I was called into office and told I was being demoted from my position and also having my pay docked, and if I had any other incidents, I would be immediately fired. I've NEVER been in trouble at work, late for work...I actually work overtime often. I was promoted through the ranks to my current position for my exemplary performance. I was shocked to hear this. I checked the employee handbook for infractions and their discliplinary actions, but couldn't find anything to justify their decision. I mentioned it to them and they said this was a "new policy". I never received any written notification of this new policy. I want to file a grievance or argue this, but I'm not quite sure how to go about this.

Are you in a union?

when you say they are docking your pay, do you mean that simply they are not paying you for the time off due to the incident?