When do you plan on going on maternity leave??

Hello, I'm 18 and a ftm and I'm very over weight. Pre-pregnancy I was 237 and 5'2" and now 250 and 30 weeks. I work at a fast food restraunt (still in high school) and my long shifts kill my back and feet and sometimes cause problems. Sunday (during work) I couldn't breathe even when sitting down it sounded like I ran up 3 flights of stairs (lasted for 3 hours) and I almost went to the hospital for it. Sometimes even after a 3 hr shift i go home and my stomach hurts so bad!! Since im so short my stomach hits everything and i have to sloach or stand on my tippy toes to do dishes, make fries, or collect/hand out money at drive thru. I don't know if i should talk to my doctor about maternity leave or what. Even one of my managers sugested it. I already have the necessities and still have my baby shower to go. My maternity leave is unpaid and I believe 6 weeks but my manager (who suggested i talk to my doctor) said they hold my job for 3-6 months. Whenever the doctor says its okay to go back. I know im at least gonna wait a couple more weeks but im just struggling. I need honest opinions and for the ones struggling like I am when do you plan on going?