Feeling very incompetent..

I am a newer nurse, working at my current position 3 mos. I actually am going to be starting with a new company in another week due to the high patient ratio that I currently have on my belt (1 nurse to 50+ residents, and at one point I was a nurse to 4 units, each having 20+ patients - which was over 80 patients to myself). Yesterday afternoon I recieved a call from the DON stating that a resident I had on the NOC shift leading into that day had presented to day shift with a pretty large bruise. To give you a visual, the unit I worked on was a locked unit for resdients with the end stages of dementia - most are on hospice. I guess when family had visited the resident stated he had fallen, but then when staff asked him if he had really fallen - he gave them a blank stare. I myself definitely know that if something were to have happened to him I would have documented documented documented - we have a "fall" program and even a system - they know that I would follow that as I have been told numerous times that I document very well for being new! My question is, she stated that the bruise is "State reportable", but seriously - there were NO injuries with him on my shift, no unusual findings, and he was checked on by myself at least 3 different times before I even passed medications in the A.M. I know they are probably asking each shift if they noticed anything, but seriously - would we really let a resident go with a bruise and not report it?? He does have alarms that sound, but the DON stated that other nurses said that he may be able to turn them off.. ok well that's weird because even though I've only had him 3 times - I have never been aware of that. I will write a statement regarding my shift, and I know exactly what to write, but am I the only one in this world as a nurse that feels terrible that my resident had an injury, but has no clue why??? I am beating myself up for this.. I never miss a beat, and now this. Any advice would be appreciated.