How to reject fatty advances

>work in retail in a small electronic shop
>enter customer who could be attractive if she lost a lot of weight
>I help her out very basic questions
>didnt think she was hitting on me at all very normal and friendly
>I ring her up
>"oh hey um could you write your phone number on the receipt in case in doesnt work"
>what the fuck I thought this happened to girls in movies
>I glance up then start panicking , I hate it when im put on the spot
>start to turn red because customers are now looking at me like their favorite show dropped a drama bomb
>I enter beta mode
>"I uh...call the store if you have a problem..."
>"well okay, you're really friendly I liked the service whats your facebook"
>I really start to panic and dont chose my words wisely
>"well...you arent really my type..."
>she looks crushed
>I should mention I have a knack for thinking and saying inappropriate stuff
>think to myself "yeah type 2 diabetes"
>let out a quick laugh
>cant resist I then burst into laughter uncontrollably
>she glances over one last time and leaves
>coworker comes up laughing and said I should have said "we cant give out personal information on the job"
>what can I say I fucking panicked

My girlfriend also has a neighbor who has Alzheimers. His name is Jack and I thought of Jack in metal gear solid 2, then I thought of when Rose said "jack remember the first day we met?" and began laughing because obviously not he has Alzheimer.
She was pretty pissed telling a serious story and having someone laugh.

Im such a dick, maybe im autistic.