A Twitter post from Lady Gaga about her diet and exercise regime has sparked a backlash from fans and eating disorder campaign groups.In a tweet to her nearly 23 million followers the singer wrote: ‘Just killed back to back spin classes. Eating a salad dreaming of a cheeseburger #PopSingersDontEat #IWasBornThisWay’With social media at our fingertips, anything can go viral before …