I fully understand your plight. In our midst of anger, we can
claim words that may/may not be right - then again, as we calm down
and think, we must learn to understand that everything happens for
a reason and we must look beyond surface problems - look at the
roots of your relationship. 
You girl obviously has some hidden tension inside her that she may
not have expressed her dis-satisfaction. Of course, its really
attitude to put her bf down when in reality she should be
supporting you. Then again, have you source for that
dissatisfaction in her? There are certain things that a woman
wouldn't talk until things happen and she will blame you for them.
Being a guy - you are cursed by nature of the inability to know
what's going on in relationship unless you are told about.
With the combination of these two - we have a little communication
problem and our relationship's 'engine' would jammed.
Learn to release that tension in her. At least, know why she is
reacting in this way. Talk not before the surface issue - you may
like to have a soul talk regarding this relationship? At least,
when this is done, you both will understand each other better. Your
definition of a supporting gf? Tell her, let her know.
Cheers
