What Causes Delay In Periods Despite Having Negative UPT?

Hi, I had recently had implantation bleeding over a week ago, suppose to start my menstrual cycle a few days following that. It s been 7 days and I m still not on my menstrual cycle. I did take a Plan B pill later than I should have almost a month ago maybe longer. I can t remember. I even took a pregnancy test but it was negative. Since it was right after my implantation bleeding stopped, wasn t sure if my HCG levels were high enough. I ve been super tired, feel more hungry but it just depends I do get dizzy and have felt like I will vomit. I also am clammy with a mild fever for 3 days now. And take phetemine.