Forgetfullness is a very useful trick our minds have to help us get over traumas. My guess is you remember the bike falling down on you because by then your mind had already realized that you'd survived the fall yourself; the most traumatic bit was over.

As for the headache and the need to nap, the thought has probably already struck you that you may have a concussion. You do have that big ding in the helmet that you don't remember getting, so you must have hit your noggin at some point. Recalling the advice my mom told me she got when I tipped my high chair backwards and hit my hed against a cabinet, sleep may be all you need to heal it. As Mom tells is, I slept 36 hours straight back then, until my mom phoned the doctor again and he said "wake her up!". So if Mr. Fish thinks you've slept too long and you still have that headache, or if you get nauseous, or just in general if you're not feeling better by tomorrow, you should maybe go to the ER and get checked out. In fact, you might want to do that anyway just to document the injuries for insurance purposes.