Depo Shot

So... I'm most likely getting the Depo shot tomorrow and I'm not sure what to expect... I've been on the pill (on and off) for a while due to polycystic ovarian syndrome. My periods have regulated themselves over the past few months... However, I'm five days late... 😣... Freaking out... Idk what to do. I took a hpt was negative... Any advice girls?!?