I see a belly bump… not sure what to tag that with.
Well, I doubt she would be giving birth to a baby Luma. Considering some Lumas say the Star Bits taste like Honey - I think she even says that in her story book via the on-screen dialogue since most characters don't speak apart from reactions, injuries and other things like in Mario Kart/Smash Bros - it could be she's been eating too many Star Bits.
Then again, she is a 'goddess of the stars'. Makes you wonder if she's actually been sitting/sleeping all that time or she's been moving around the galaxies.
Otherwise, she's just lonely and wants someone to be with.
Sorry for waffling on - I'm just a Rosalina fan ever since she made her debut in Super Mario Galaxy 1.