When I pipe my cake for decorating, some have a complex design so I take at least an hour. But after a while, the whipped cream started separating in the bag. Is this because of the warmth of my hands?? I have no idea why this happens. 
Are you using an insulated piping bag with a disposable plastic piping bag inside? Is the room hot? Is the whipped cream over whipped? To solve the heat problem, try folding a dissolved gelatin sheet into the whipped cream. If the cream is over whipped it will start to look a bit grainy, as the fat starts to glob together as the fat turns to butter. 