Android : Why is the facebook api erroring out? "R is not found" in android

I am working on an android app in android studio that includes facebook. Everything was compiling and running just fine, but then suddenly I am getting hundreds of errors in the facebook code. The errors are

"Error:(29, 28) error: cannot find symbol class R" and "Error:(29, 28) error: cannot find symbol class BuildConfig"

These errors are not related to my project (except that my project uses facebook login).

I have tried: 1. A "clean" build - did not fix the issue 2. Restarting my computer and "cleaning" the build - did not fix the issue 3. Upgrading the facebook api to a more recent version (then downgrading it back to the original version) - this wound up with the same issues for both versions

Im not sure what the issue is or how to solve it. Any ideas?