Is it normal for girlfriend to be obsessed with my bellybutton?

While it does not really bother me, my girlfriend always tries to poke my belly button through my shirt multiple times a day. When we are lying together, she lifts it up to play with it, and sometimes, fingers it for a couple minutes. She has also used her tongue. Is this normal, and does anyone else do this / have a girlfriend that does this?

Is it normal for girlfriend to be obsessed with my bellybutton?
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