sore round the belly button anyone know why?

Chelsie • My 1st born girl 🎀💗 My angel boy son 👼🏻💙 & My rainbow girl on the way due 5th oct 2017 🎀💗
I'm 2 days away from being 24 weeks and it's gets sore round my belly button from time to time. It's not a constant thing & it just happens every now and then, I'm wondering if it's just the muscles growing or my skin ste thing as I'm getting bigger. Does anyone else get this? Or know what it is?