Please help, I'm so scared and this is all I've been thinking of for the past few weeks.
On April 16th, my boyfriend and I had intercourse, I am not on birth control (this was one of my first times ever being active) but he did use a condom. However, the condom slipped off. He stopped as soon as he noticed and he did not finish inside. We had intercourse again the next morning with no problems with the condom this time. I took Plan B about 11 or 12 hours after, with no side effects. Six days after taking Plan B, on April 22 I started to bleed. At first I thought it was my period but I noticed it was much lighter. It is now May 2nd and I'm still very lightly bleeding. It's definitely not my normal period because it is a much darker color and very very light in comparison. I know that no birth control method is 100% effective, but should I be really worried about pregnancy?
I should mention that for the most part I'm very regular, except starting this year (2017) I was almost 4 weeks late on one occasion (I was still a virgin so I wasn't worried) and the next month I was about 11 days late.

I would really appreciate any help because I can't concentrate on anything else :/