Accidentally rude to someone because I thought he was making fun of me

When I was in school I used to get asked out a lot as a joke/dare or would have people pretend to be my friends or invite me to stuff as a joke or like have a competition of who could get me to fall for it and accept their invitations. I'm 20 now and at uni and obviously I'm aware it would be seen as very unusual and immature for people to still be doing that, but whenever someone shows interest in me, my first thought is that they are joking or it's a dare. I wouldn't say I'm ugly but I was an easy target for bullying because I have always been quiet/an introvert and was extremely timid and unwilling to stand up for myself in school.
The other day a guy came up to me when I was leaving the library and said he's seen me in lectures and thinks I'm cute and asked if I'd like to get coffee with him sometime. My immediate thought was that he must be making fun of me so I instinctively laughed and replied "are you joking?" and just walked off.
Obviously, looking back on it, I realise that he probably wasn't joking and that my response came across as extremely rude and if I was saying "you must be joking to think I'd be interested in you". But I wasn't, I was genuinely and sincerely wondering if he was asking me out as a joke. Obviously, again, I realise people don't really still do that at 20 but my first instinctive reaction was that he was making fun of me and I should respond in a way that shows I'm not falling for it and am laughing along so he doesn't get the chance to humiliate me.
Now I realise that it probably wasn't a joke, I can't stop feeling bad about it and feeling like I made a fool of myself or came across as a stuck up *****.