Scarleteen

What does it mean when my bellybutton hurts after I masturbate? (I have not been in an intimate relationship and I never inserted anything inside myself...)

Probably nothing, and also probably a complete coincidence. But any kind of ongoing physical pain is the kind of thing you need to bring to an in-person healthcare provider who can examine you. We can't substitute for that kind of care here.

Also, sometimes when people masturbate or have other kinds of partnered sex and don't reach orgasm, they can experience some pelvic discomfort for a little bit afterwards. That usually passes on its own, but an over-the-counter pain reliever can also speed up that process.