Correct me if I'm wrong but are you suggesting that Jill and Bella are or were a secret couple? I don't believe that's the case, my theory is that Bella is Jill's sisters friend and the reason why she is so overprotective is because she is doesn't want something bad to happen to Jill, because like
Tserriednich'sNen pointed out she has been on many dates and Jill also said the guys that approached her didn't approach her because of her but her family. Jill was obviously hurt by that which who knows maybe Jill shutdown and was depressed because the guys that wanted to date her wanted to use her. This is where Bella comes it, like I said earlier I believe Bella is Jills sisters friend and she prob promised Jills sister to take care of her. Bella prob saw what happened with the dates to Jill and got pissed off and decided to protect Jill and not let anyone hurt her because she loves her as a best friend not because they are intimate.