The longer you date and get to know each other, the information you reveal to each other becomes deeper and more intimate. In addition, the longer you date someone you sort of become privileged to some information about the other person, like how many boyfriends or girlfriends they've had, maybe the name of the person they have been in love with before, or even the number of people they have had sex with.
But the important thing about sharing this kind of information with your significant other is that you trust them enough and they care about you enough to not judge you or hold that information against you.
Bottom line...it's awkward. You may not be expecting it and get caught off guard by the question. I can't tell you why the other person made the leap into seeking information about you that you aren't comfortable telling them yet.
Maybe he or she is insecure about your status with them or they are in new territory with you and don't know when the right time is to share certain information.
Either way, just because they asked doesn't mean you're obligated to answer. You can tell them you're not comfortable talking about that yet, that it's something you can tell them a little later, or, depending on how long you've been dating, you can flat out tell them it's none of their business.
If they push you to reveal whatever the information is, you might need to ask them what the deal is and why they so badly need to know that information.
Once you put something out there, you can't take it back. So make sure you're good and ready to put it out there.
xoxo Nickie