Jen Selter -- fitness model turned ass Internet sensation -- tells TMZ Sports ... she LOVES when famous athletes and celebs reach out to her about her butt 'cause "it makes me feel special in a way."

Selter and her bulbous derriere were hittin' the streets of NYC when we talked about how her 'donk has exploded on Instagram ... with more than 2.5 million followers ... many of whom are bigtime stars.

"It's cool when talent reaches out to me ... 'cause it's like, I feel special in a way."

Also included in this clip:

-- How many squats Jen does per day
-- Who's got the best butt in Hollywood

The end.