I've never had this happen before but my fondant is bulging underneath my cake!!! It's been covered overnight and it's like a massive air bubble has formed - under the buttercream underneath the fondant. Anyone every had this happen?

Yes--

A potential fix it type idea is to try & poke it with a pin aroung the edge where it won't show too much to let the air out & see how it goes for you. Sometimes it wrinkles and sometimes it doesn't.

There's tons of theories about why this happens.

My experience is that of Forest's, "schtuff happens'.

We've tested this every which way possible and it happens to all kinds of cakes anf icings no matter how it's handled in my experience.

I saw it happen once while I iced the cake.