Help.

I didn't put my crumb coated cake into the fridge and after I laid the fondant on it, a bubble of frosting appeared in the middle where the 2 cakes connect.  What did I do wrong and what should I do?

Thanks.

Try and take the fondant off. If you scrap off the bc, you can't still use it again. 

It sounds to me as if you used too much buttercream between layer. Or it is to soft. Or both. Most likely it's the first (too much). Squeeze down on the cake to get rid of any access and re-ice  again. Fondant on. Done.

Next time place a heavy book on the cake after you put your filling between layers (but before crumb coating). The book will imitate what the fondant will do later : applying pressure through added weight on the cake. 

Sorry it happened to you and hope you have enough time to fix  it.

yes, definitely left the tier settle with a weight on top.  However, for the bubble right now, if you have a coffee stirrer, stick that into the bubble and suck out the air.  (Don't blow and you're not contaminating the cake.)  Then repair the hole from the stirrer with fondant glue, or put a deco on top of it.