Ok so I have had this problem few times that I smooth the buttercream very nicely on a cake, put it in the fridge for the night and in the morning I cover it with fondant. In the beginning the cake looks great straight edges... but the more I work with it in the middle or I guess the places that the layers of cake are placed the fondant moved inside. So when you look at the cake from the side you don't see a straight profile, you see a curved profile.(Like a C on the right side)

And obvious;y the more I work the worse it gets...

Any ideas? I hope I explained well??;-)

Well, this is what I usually do not sure if this will help your situation but here it is any way. I usually bake the cake let it settle overnight, next day I fill the tiers (don't overfill) wrap them up in saran wrap and place them in the fridge with a book on top of each(about 1 LB) then the next day I ganache or butter cream with no problems with shifting. If my cakes are over 4 inches in height I usually just place two dowels near the center to further keep from shifting. 

Hope this helps!

AYou might also be using a thin consistensy buttercream/frosting. And your fondant might be rolled too thick..

Both of the previous replies have some helpful info. It sounds like what you're describing is known as "cake bulge".  As stated, filling, buttercream/fondant consistency, doweling issues...etc can all play a part.  Without knowing more about your techniques and construction, it's hard to say exactly what's going on with yours, but maybe do a search here on CC for cake bulging and see if any of that information helps :)

Thanks so much for the replies!! I will definitely look into cake bulging!

Ahttp://cakecentral.com/t/633571/my-newest-trick

Heres a good thread about helping get rid of cake bulge...hth..

AI have stopped using buttercream on my fondant covered cakes for that reason. I now use white chocolate ganache. The ganache holds up to the weight of fondant really well. Also, your fondant should be rolled really, really thin. I like mine to be about 1/16" of an inch. Once rolled that thin you can almost see though it.