Strange Nipple

[PressPaws]

I noticed while giving 4 year old DD a bath today that one of her nipples was inverted. When I mentioned to DM she said she'd noticed the same thing while bathing her a few weeks ago. Prior to this it has always looked normal. Checked not long ago and it looked normal again so it seems to be coming and going.

I've been googling and scaring myself about possible nasty causes (I have bad health anxiety at the best of times and really shouldn't google). But there isn't much info about this happening suddenly to young children.

I'm just wondering if anyone else has noticed this with their young DC? If it's a common harmless thing I'll feel silly for worrying but will also be hugely relieved.