gosh this is just awful - either way it presents a dilemma - how to handle bullying or how to handling lying.

Perhaps having a conversation with her about options when the incident started - go tell the teacher - find other friends to play with - what to say to defuse a bully - all that kind of stuff might clarify what actually happened. We had some scenarios with DS 4 - but what were you doing before he hit you - oh I knocked down his block tower. Your DD is older so I doubt its that simple but perhaps you could get to the bottom of the nagging feeling by talking about her day prior to the incident? But it does sound like she is really internalizing something - so maybe some extra doses of cuddling and not doing anything or reading together, whatever she likes to get her to open up. This is so tough!!
Good luck and sending some cyber hugs!
DeHe