Oh Gutted dont listen to people when they say to think of someone else... that is emotional cheating and you dont want to do that. Besides this has nothing to do with whether or not you think shes hot lol...
Sweetie your body might just be going through a change. Maybe you need to find new ways to please your body?
Also, sometimes our bodies just arent ready for sexual activities! Dont take this to heart cause its not a big deal I promise you...
Let her know its not a big deal... turn it into a joke if you have to to lighten the mood..

Youre normal! Dont worry!