I'm sick but not allowed to miss my shift

Last night I developed an awful sore throat and headache and couldn't sleep. This morning I have a fever (102F), body aches, an awful sore throat (can't speak), the same awful headache, and lots of phlegm. I have a shift at the animal shelter from 1:30 to 4:30 pm but I feel like I'm physically in hell and have been crying all morning. As volunteers, we are only allowed to miss 3 shifts, and I've already missed three. Otherwise they'll "fire" us. I don't know what to do. I tried getting a doctor's appt but they're closed on Saturdays. And my boss is a bitch.

UPDATE: I have strep. It hasn't improved days later and I'm medicines aren't bringing my temp down. I went to urgent care, they did some tests and prescribed 850ng amoxicillin 2x/day for 10 days. My dad drove to the shelter and explained that I was ill, and they were kind to him. I have the best dad.