If I took a plan B pill 12 hrs after sex and haven't had spotting/side effects does this mean sperm didn't get inside me? (4 days following the pill)

No. plan B is only 59 - 94% effective at preventing pregnancies. This can be affected by the woman's weight, where u r in your menstrual cycle, and if u have unprotected sex afterwards. I don't think there is a way by symptoms to tell if it worked (until u start 2 get symptoms of pregnancy, that is). The safest thing 2 do is 2 use a condom every single time and be on a 2nd form of BC such as the pill.