Have I been discriminated against by my employer if I was the only one drug tested?

I work for a company that provides valet service for a local hotel. On Easter, while working, I went to the parking lot to pull a vehicle up to the building for a guest who had requested it. As I was leaving the parking lot, I was suddenly hit without any warning by another valet worker who was also pulling a guest's vehicle up to the building. He was pulling from a parked position. My shift was over but before leaving, I asked my supervisor if he needed to speak with me before I left. He told me he was busy taking care of the guests, the paperwork involved and we would talk later. After I left, the police were called to take a police report. I was never told this would happen and I needed to stay. Less than ten minutes after I was home, I received a call from my supervisor telling me an officer wanted to speak with me. He asked my version of what happened and I told him. The following day was my day off. I received a call from my supervisor telling me that I would have to submit to a drug test before I could come back to work the next day which I did. When I returned to work the following day and brought the accident up to my supervisor to discuss it with him. I could see I appeared to be the one they were blaming. I asked him if they watched the video from the security camera overlooking the parking lot. He said that the camera doesn't get that part of the lot. The camera is a state of the art and see's in all directions at one time, and is approximately 50 feet from the area where the accident happened. I later sent an email to my supervisor expressing my concerns and asking if the other worker was also required to submit to a drug test. He replied that we will discuss this later. To date, he has never discussed my email with me. I decided to do some checking myself and learned from two other individuals that the other worker was not required to take a drug test, only I was. That is obviously why my supervisor has never discussed my email with me.