The Mane 6 are infected by Lust. One of the 7 Deadly Sins. Why? They're not sure. Was this Discord's doing? Maybe. Are they going to have fun with it one way or another? Certainly. Do they all feel the effects differently? Somewhat.

For reference and to keep the story itself straight: Lust, in this version of the MLP universe, was a creation of Discord. Along with the other 6 "Deadly Sins", Lust was unleashed to wreak havoc across Equestria. When the 7 Deadly Sins were found, they were sealed away inside stones scattered across Equestria. Lust's stone was unearthed on Pinkie's rock farm while Cloudy Quartz was pregnant. Lust infected the unborn Pinkie Pie. The others have yet to be decided. Discord forgot about the Sins, much like he forgot about the Plunder Seeds from season 4.