What Causes Delay In Menstruation Other Than Pregnancy?

Hi, It would be greatfully appreciated if you could help me, I am 22 years old and my last period was 13th september, ive took about 4 pregnancy tests and have all come back negative. Then on Tuesday 9th of november i went to the toilet in the morning to find that when i wiped myself there was a browny coloured substance, after wipeing there was nothing else there but to be sure i put a tampon in, Later that day i went to toilet and checked tampon to find next to nothing there again. After this there was nothing apart from next day when there was a lighter couloured browny discharge there, Not even enough to use a tampon. From then i have had nothing else. I also took a pregnancy test today which is friday 12th of november, Is there any chance that this could be implantation bleeding and i have tested too early as im lost with my ovulation dates because of not having a period since september 13th. Thank you for your time.