I guess you need to see the love doctor then?

While we're assuming things, let's just assume they're being wishful OP ;)

Should've responded, "I'm assuming you're a bit of a bitch."

But are you actually single? It'd suck more if you aren't..

I had this situation when handing in my tax report. The cute woman behind the counter opened a box of paperwork, leafed through, then looked up and asked "Are you single?" to which I replied "Yes..." She then clarified that because I'm single, not married, I need this form instead of that one. Wishful thinking.

Should've replied with, "yes, I'm free Friday, if you're trying to ask me on a date?"

She probably meant single in terms of marital status, not in terms of whether you're in a relationship or not. If you're under 25, that's not an unreasonable assumption to make (although I am 23 and married, so who am I to talk). She probably needed it for a form. I assume "are you sexually active" was a separate question.

You coud always have asked 'why?' - if there's no good reason, you could always change doctor

"For you, I could be".

Fire that doctor. The last thing you want is a doctor who makes assumptions- you don't want her to assume that you're being a crybaby about menstrual cramps and send you home with festering acute appendicitis.