I'm Wondering If I Should Take Plan B. On Sunday

I'm wondering if I should take Plan B. On Sunday (around 6pm),I had partial penetration (just the tip) occur accidentally on Sunday for a few seconds. I was on day 30 of my menstrual cycle and I have a cycle of 35/36 days. It is almost 3 days later (nearing the time limit for Plan B) . I am currently feeling symptoms of my period for the last 2 days (backache, cramps, peeing a lot, etc.) I believe I should be getting my period on Thursday or Friday. I don't know what the chances are that I may become pregnant. Should I take Plan B?