I think it's because you had a superficial crush on that guy and it went away because it wasn't fed.
When you saw him, it always woke up the 'thing you had for him' inside you and thus it 'feeds itself'.
Because you didn't see him for a long period, those feelings were never touched or woken and because of that, it diminishes and diminishes until there's nothing left.
A female friend of mine was in a relationship with a guy for about 4 months, when the guy went on a vacation for a month, she didn't feel anything for him anymore.
So don't think this makes you weird or anything as this is normal, also for NTs!