I would allow that you could continue to crawl if you wanted to after being knocked prone and still had movement points left, but I'm not excluding the possibility that some rule might exist to prohibit this.
A PC moves, at some point in that move provokes an opportunity attack, gets hit by the opportunity attack which slides the character.
How does one handle this situation?
Move aborted or continued after resolution of slide?
Does the PC get to change his destination before continuing the move...