From what I read at several places, Erin may seem happy and having fun at first at the party, in fact, it didn't last, she got upset with all the BB talk and stuff and ended up leaving the party early in tears and Will DID NOT go after her and continued with the party and talking and laughing with people and whenever Janelle's name came up in convos, he LIT UP!!
Give it time, they are headed for SPLITSVILLE!!!!