Yeah, I agree with the above... you could have just straight up said to the patient "I don't believe what you are telling me" and then the ball is in their court. Then can just get pissed off and say "I TOLD YOU, I FELL!!!", or they can tell the truth, that someone beat the crud out of them.
If the patient was indeed fully competent and oriented x3, and they stuck to their story that they fell, then I don't know if you are *legally* obligated, at that point, to report that you suspect abuse. But you probably might have wanted to at least confront the patient and let them know that if there were anything more to their story, you wanted to know about it.
You are new and you still have a lot to learn. Stuff like that is hard. I am a new psych nurse and I have to ask all of my patients if they are being abused or if they have been abused, and it's still hard. Actually, I had to ask all of my Med/Surge patients similar questions as well... it's a hard subject to approach, but one that we as nurses have to learn to face and help our patients deal with and get help with, as much as possible.
Good luck. Like I said, you are still learning the ropes. You'll do fine.