Medication Problem Please Help

I have been a nurse since January. I was placed on suspension until monday when I meet with my manager and human resources. I was covering for a nurse at lunch. I gave one of her pt pain medication. When she came back from lunch her pt claimed she never got any pain medication from me. I went into the room with that nurse and told her this is the pt I gave the med to. Anyway, she put in an incident report even though nothing happened. My manager made me sign a paper sating that I am on suspension due to medication administration discrepencies. So now I am wondering if when I have been pulling my meds and i turn around to put them on the counter and someone says hey can I grab something real quick they aren't signing me out and they are grabbing things under my number. No matter what it is my fault. Does anyone have any advice??????????? I would appreciate anything!!!!