Belly button fondling?

what does it mean if a girl randomly wants to play with your belly button?some girls really seem to enjoy this,i dont have some kind of an abnormal belly button i just have a round belly not some huge flabby thing.it just feels strange and makes me quite uncomfortable but its arousing at the same time.its like being molested in public like i could imagn what a girl would feel like if it was socially and publicly acceptable to playfully insert my vinger into you know where...its just strange do many girls do this with guy friends?or does it mean something more?it might be important to note that these are girls that i usualy like more than friends,are they flirting with me?