how do u know if your gf has lost intrest in you??

me and my gf have been dating for 2 1/2 years now, for the first couple of mnths we didnt have sex, we only fooled around. then we started to and had lots of it, for awhile. Now we only have sex about once or twice a month. i've asked her if we are ok, and why we dont ahve sex as often. she said shes really bussy with school, and work... so how do i now if she lost intrest in me.