Plan b

So I had sex. We used a condom but it slipped off inside of me. So an hour later I took a plan b one step. It's 2 days later and I don't think I've had any symptoms related to the plan b. How do I know it's working? I'm really not trying to get pregnant, my ovulation is in about 4 more days, will I be okay?

Update: I started having back pains a few days ago and yesterday was cramping a lot and I started bleeding. So hopefully this means I'm not pregnant! On April 29th I believe that I was on my period previously. I no longer have the intense back pain or cramps.