Missed period, but i’m not pregnant

So i had sex yesterday, but my period was suppose to start the first, so it’s been late. Yesterday was the first day i’ve had sex since november. I’m on the depo shot & he didn’t cum in me at all. I’m stressed that i am pregnant, even tho i’m sure i’m not bc my period was late before i even had sex.

Any ideas? Should i be nervous? Should i go buy plan b on monday? Should i stress about it?