Can you help me???

Was it bulging where your filling was along the middle of the cake or all over? Sometimes, if I don't put a thick enough dam for the filling and my fondant is too thin it will bulge around the middle.
 

No, before you put your filling between the layers put a thick piping of very thick buttercream around the edges of the cake. Then add your filling and make sure not to put too much in.

Making the fondant thicker will probably help too, but if you aren't using a buttercream dam then your filling is seeping out and causing the fondant to dimple.

Yes a dam will prevent that AND putting less filling inside the dam.  I find that airbubbles form under fondant when my cake filling has air holes in it - in other words, I didn't put the filling right up to the dam in spots.  This causes air pockets and your cake has to get rid of the gas - like when we eat beans :-0!  It's gotta come out sometime.  Some ladies on this board swear by placing a weight on top of the filled cake over night to squeeze all the air out - before crumb coating then finish coating it. I haven't tried this but will some day.

I'm slightly confused by the dam, would you fill the cake first and then cover with buttercream?

Here's a mental picture that might help you out.  Each cake you bake consists of two cakes sandwiched together with filling.  So you'd have your cake plate, on that one of the cakes then rim the top of that cake with a dam of icing then put your filling inside the dam the put the second cake on top of the filling - give it all a thin basecoat - pop it into the freezer for 10 minutes to harden then apply your final coat.

Wilton has basic instructions for all aspects of cake making for those of you just starting out. Go to their website and read the basics. Here, it explains a buttercream dam:

http://www.wilton.com/cakes/making-cakes/filling-cake-layers.cfm

Cakes by peeps:  I don't use thick b'cream.  I have always used my b'cream in medimun consistency.  Many seem to think it needs to be thickened by adding more powdered sugar &/or cornstarch until it is the consistency of pie dough!.  What workes best for me is to pipe my dam using tip 12  - not an open coupler/one w/o a tip as that makes too big a dam and you end up using more filling which ends up seeping out.  If you use a bit less filling (especially if it is on the loose/thin side) you shouldn't have problems.  If you want more filling then torte each layer making space for another thin layer of filling.  Also making *SURE!* your layers are level help. 

Oh, forgot to mention some use fondant instead of thickened b'cream to make the dam.