It may be hard to see in the picture but it's like the oils from the BC are separating and seeping out the bottom! Where did I go wrong?

This happens in heat and humidity.

In this weather I don't add water/milk to my buttercream.  I make what is considered stiff consistency, but in this weather it isn't stiff, It's just right.

I just made cream cheese icing and added no milk.  It's humid here today.

What kind of buttercream did you make?  Crusting (with or without butter) or SMBC/IMBC? 

Yeah, it's pretty humid and hot out. I just thought since we had the ac on it wouldn't affect the cake like this. I don't know if my buttercream is crusting or not. How do you know? I used butter, crisco, vanilla, milk, and powdered sugar. Should I be putting meringue powder in it? I think my ratios were also off--I was making a lot of frosting and doubling and I think I used too much crisco. I should have known because it was medium consistency at best.

I made another batch yesterday and used more powdered sugar and the correct amounts of butter/crisco and it held up, but only slightly better. It didn't "run" but started looking wet/glistening. I am definitely going to try no water/milk the next time-thanks! Any thoughts on the meringue powder?

The meringue powder will stabilize the icing.  If you are using 1/2 Cup shortening & 1/2 cup butter, try using the meringue powder and 3/4 cup shortening and 1/4 cup butter.  It will hold better in the humidity.  I have central A/C too and I still need to make adjustments in the hot humid weather.

Good luck.  It is the butter in the icing that is melting first.  So use less until the weather cools down.  That is what I do.

Thank you so much! I really appreciate the advice!