Workplace Horrors

Working up at the school (janitor) I’ve worked there for 21 years and never noticed anything until that night. Something happened one night that I’ve told my family over and over for years. The paranormal does exist.
I was working in the Tech wing one night up at the school such as I said before, minding my own business and cleaning the rooms of each class. My other coworkers had finished before me, letting me know where I had to clean certain rooms and reminding me of locking all doors.
I was finished at 9 sharp, pressing the button on the elevator going to ground level. The doors closed I was going down but as soon as the doors opened again, I was on the same floor. I thought it was some mistake,that I pressed the wrong button. I looked out into the hall the notice a door was cracked open with light seeping through. It was the chem lab so a light must be on at all times (a small night light) but the door was not supposed to be opened.
I thought I must have forgotten to close the door, so I walked down the hall to close it. I reached the door and pushed but as soon as I pushed something pushed back not allowing me to close the door. I was startled so I was going to turn on the light. but as soon as I reached my hand in I felt a dark cold presence coming from the other side of the door. I reached my hand back as quickly as possible and kicked the door shut. I ran to the elevator and pressed the button to get to the ground. The elevator moved down, but when it opened I was once again on the same floor. I ran out of the elevator I couldn’t possibly make that mistake two times, onto the landing and down the stairs.
Telling my co-workers they all found me nutty until another night one other coworker felt the same thing. whatever it is I hope it will leave us alone.