It's a bad idea to ask about rejection in general, whether it is after a few days, weeks or even years. People who reject you are hardly, if ever, honest about the reason for rejecting you. It would be pointless to ask. Just assume that they were not attracted to you.
Not really a bad idea, but I'd have asked it the same day if I were you (ask why via text/online perhaps). How many weeks ago was it to be precise?
You can maybe initiate with saying: "Hey, I have a question. Remember that time when I asked you out? Why exactly did you reject me? I just want to know this so I can gain some insight in what women like and stuff. It will definetely help me out!"