Is it normal that i completely ignore people after knowing them?

I am a very awkward, shy, insecure person. I am okay at making aquaintances, you know, causally talking to people when im taking same classes with them and etc.

But there is this problem i have

I completely stop talking, say hi, or even acting like i know them when i stop seeing them during summer break or stop having same class/es. I just feel really really awkward.

I guess i feel like im not THAT close with them, and get tired of saying hi to everyone whom i knew in that one class. I just cant help it... Could it be offensive? Does anyone else do this?