(Warning: huge spoilers for Riverdale ahead. Read at your own risk, fans!)
While these spoilers may have ruined the surprise of meeting a full-on pregnant Polly, the show has actually dropped a ton of clues about Polly and Jason having a baby. Jason selling drugs makes a lot more sense: perhaps he needed money in order to support his child. It also explains why Jason's grandmother told Betty that Polly and Jason were engaged — perhaps Jason proposed when he learned Polly was having his kid.
While Hal made it seem like Polly tried to kill herself over something that Jason did to her (which could still be true, we don't know yet) it's far more likely that Polly was sent away for as long as she was because her family discovered she was pregnant. Could this reveal about Polly also explain why Jason wanted to run away from home? Maybe Polly was sent away because her family worried that she would try to run off with her much-detested boyfriend.
This spoiler is a juicy one, but while it provides some answers about Polly, it also opens up a ton of questions.