the difference between stalking and really interested?

Stalking means crossing boundaries. Set some clear boundaries about what you found acceptable. Communicate these boundaries clearly. If he crosses them, then he is stalking.

Anything that he does that you find acceptable doesn't count as stalking. If you have any kind of dating relationship with him, then it is your responsibility to communicate clearly what you like, and what you find unacceptable.

There is nothing wrong with making it clear there are questions, particularly about your past, that you don't feel comfortable answering.

You answered your question here.

If you're gut is telling you something, listen.

What your gut is, is the part of your brain that acknowledges somethings wrong, but you're afraid to say it because society has imposed various "rules" you're supposed to obey.

I think it's taking it too far in if you ask him to stop and set some boundries on him and he crosses him. I think that sometimes we can be too polite and not want to hurt their feelings, but in doing so only pent up our feelings and get frustrated inside.

But on another note, if he gives you the creeps, flat out tell him no.

When you meet someone and it just doesn't click,
in fact he gives you the creeps, then move away. Don't make eye contact, talk on the phone or indicate that you even want to be a friend.

If need be, say "I'm sorry but I just am not able to (call, date, talk, listen) at this time."

You say he asked questions. What questions did he ask that creeped you out?