every girl fakes it at some point, guys really cannot tell! or maybe you don't care!
sometimes it is better just to fake it & get on with the show tho. mood, stress, distraction, hormones- these things that are frequently changing in a woman's body, more than a guys & can mess with sex drive (maybe for a minute, maybe years!!)
personally i think the sex can be great even if i don't get off at all. IF it's fun & good otherwise & sex can still be just as great! even an unsatisfactory man, small one, whatever isn't working- can be settled myself For Real with my own fingers