S.B.
When my belly button popped out, it didn't go back in until my babies were born, but since you just noticed it today, maybe it was the way your baby was pressing on your tummy from the inside.
I don't think it's anything to be too concerned about.
I know people whose belly buttons never popped out at all.
It's just one of those things.
Take care of yourself!
Be sure to let us know when the baby comes!