If I were you, I would've tried being anticipatory on what's wrong with your chicken (The tarp being an obvious thing if your area is prone to rain). You seem very young and I'm disappointed through reading through all of this it doesn't seem like your parents stepped in to help you feel like you had support to care for her.
If I were you I personally wouldn't get anymore if your father's going to raise hell over the idea of a sick animal needing care by coming into the house. And while I realize putting it all on you while emotionally unfair, is even more so to the chicken who expects you to be her advocate and care giver. I don't think you're a terrible person, and I am glad you're still trying to keep her alive and comfortable, so please know my displeased tone is directed entirely at the fact that you're railroaded into dealing with this entirely on your own in real life with the exception of us on the internet. It's been long enough though if you have meetings to go to and what not (I assume afterschool clubs or a part time job) You may need to knuckle down and spend the money to take her to a vet and take their advice directly. They may even have something medication wise to bring down inflamation or help her recover faster if it is at all possible.