What the heck!?

okk! so this is my situation. I took plan b as a precaution after getting pre-ejac. on my . everything went well, I had my period or what seemed to be my period 6 days later. now it's 20 days later and I saw a single spot of light red blood on my panties. I also want to mention that I took a blood pregnancy test exactly 14 days after taking plan b and after having a period. Should I be concerned or are my hormones still messed up? Should I expect my next period on time? I haven't had any blood after it came on my panties today. so please if anyone has had a similar experience or can shed some light on my situation, I'd be forever thankful!