Well if you can't remember doing anything stupid then you can't really blame yourself can you? (though that doesn't stop a mother....)
I think its very honest of you to say that you don't know (it would be easy to rationalise it to yourself and just automatically blame the other party). Your best position is not accept or cast blame under these circumstances.
What does the other person say happened?
Glad you're not too badly hurt though, and I hope there aren't any repercussions (other than motherly wrath).

Kris