There is no way this happens to you.  Your girl can't be, but at certain times there are some women who actually pretend to have an orgasm. According to a recent survey 54%  of the women asked admitted to faking it.   Why?
For a long time, many thought women faked orgasms during sex to protect the fragile male ego from the disappointment of failing.  Now researchers have concluded it has nothing to do with that.

Dr. Farnaz Kaighobadi of Columbia University, conducted a new study that has found that faking orgasm is more closely associated with how insecure a woman is concerning her relationship.  Basically, the more she fears her man may stray, the more likely she is to be a great award winning actress in bed.

Dr. Kaighobadi explains;

Pretending orgasm may be part of a broader strategy of mate retention available to women to maintain their intimate relationship.

He goes on to say that its evolution that causes women to fake an orgasm.  Previous research found that the purpose of the female orgasm may have been to retain sperm from genetically superior mates.

Over time, female orgasms would have become signals to males that they have been chosen as mates. Eventually, this way of signalling mate selection may have led to the propensity to fake orgasms.

So if you find out that your girl is actually faking an orgasm, don't get angry and let it destroy your fragile ego.. but  blame it on evolution.