What Causes Missing Periods And Spotting?

Hi I'm shonnah...I'm 27 years old. I'm concerned that I may be pregnant my period was due on the 26th of april and I missed it and I never miss it. I went to the bathroom. N I have some spotting its brownish n a lil ceamping blood n pregnancy test negative but I'm sure about that because I've had negative results and I was pregnant. I'm having all pregnancy symptoms but am unsure