Re: Very top of penis exposed.
Others said what I would. I agree with what's been said. The only thing I would add is more about specifically what happens.
As you must have noticed, the foreskin doesn't just rub onto the skin beneath it, it's actually rolling and unrolling in a fluid "gliding" manner. Sometimes it can get positioned so that more is rolled to the inside than the outside. This can make it sit further back than usual. That is what happened, isn't it?
This is nothing to worry about. It doesn't hurt anything. It's also really easy to fix if it's bothering you. All you have to do is retract the foreskin and reposition it forward again. It usually goes back forward the right/normal way.
For me, it only gets positioned this weird way if it catches (in an unusual way) while rolling forward.