C O N F U S E D

Hello ? I used to have regular menstruation but this month, 18th of March I experienced spotting (dark brown) and in the next day,19th of March  is a light flow period, on 20th its a spot again. I had cramps for almost a week started on 18th and a leg cramps. I never noticed that my breast is swollen. I tried already 2 pregnancy test but it says Negative, do you have any idea what is it?