I agree that Ashley is the most likely, and if she's pregnant it would explain the whole "It's complicated" reason for her to not get back together with Derek right away.
Sally and Heather are not random, however. Tommy and Heather recently argued, and her period was one of the points brought up (and she bit his head off). Sally is having trouble with Magnar. This is most evident when in the midst of a major dorm party Mag is burying himself in books asking to be left alone, and Sally is out partying. She might have said something to him at one point, like a simple "Hey, I'm late". Being paranoid about his girl being knocked up would absolutely stress the boy out of a party mood.