If she can't remember being hit i'd be concerned that she hit her head and may have a concussion...in which case she shouldn't sleep and should go to the ER immediately. Concussion symptoms include swelling around eyes, vomiting, fever, spaciness, difficulty tracking with eyes. I'm not a doctor or a nurse but I would take her in tonight.
If not a concussion, something else is going on and I guess i'd want to take her in tonight because I would worry.
Sorry I don't have any other ideas and I hope she's ok:(
Edit*** I just wanted to share that after reading the other responses it became blatantly clear to me that I'm a first time mama! Lol! :) Perhaps I was a bit too alarmed. I was going to erase my answer but thought some of you might get a kick out it because I do think my worry is something I am adjusting to as a new parent...and it's kind of funny:) I think the other mama responses are probably more on the mark:)