had the exact same experience several years ago when a pit bull attacked me riding. Was going 20mph, next thing I know I am standing with cuts and bruises all over, trashed helmet with deep lacerations to forehead and a broken 1st rib ( which indicates substantial force). Zero memory. On the upside, it made me realize that if I ever undergo a really serious potentially life ending accident, there is no perception of pain or the event.
I have cared for hundreds and hundreds of severe traumas and to a person, not one recalled the event.
Good luck and get better.