Question Mark is the nineteenth story arc in EGS:NP. It started with EGS:NP Comic for 2015-02-23 and ended with EGS:NP Comic for 2015-03-27. It's canon and takes place immediately after Squirrel Prophet. Immediately after - at most few minutes later.
Story deals with Sarah and her just discovered mark: she tried to find what the mark does on her own, first taking the shape of mark as a hint - "good at trivia", "good at math" - then whatever ideas she have - "turning into a guy", "flying", "werewolf morph" ... it's quite likely she tried lot of other stuff as well. Then her sister Carol visited and knocked just when Sarah is naked ... and without knowing it, Sarah casted the spell and found Carol frozen behind door. Then everything reverts, she's back naked and Carol is still knocking.
She talked with Carol, verified that she spend more time in that "frozen" state that passed in reality, said she needs to check something in her room before going on dinner with Carol, and behind closed doors tried the spell again - apparently, since she already casted it, casting it second time wasn't problem. She played with Carol's hair and then accidentally found another ability of spell - she switches body with Carol. Of course, nothing happened in reality, everything is just in the magic snapshot (or as Sarah called it "Magic virtual reality time freeze"), but she's weirded enough do stop experimenting.