How often u used to have sex with your gf when your best fren was not with u?Don't tell me u never had sex with her.If she feels less interested to you compare to past then there is something goin on.
Don't feel too much insecure.If she truly loves u then u don need to worry but if u both are in [Disallowed String for - Bad word 'fuk']a [Disallowed String for - Bad word 'fuk']a relationship screw her and find new one.