What Causes Brownish Spotting Despite Having Negative UPT?

Hi.. My LMP was February 17th with an accurate average of 27 to 28 days cycle.. on my expected day of period, I had spotting only while wiping and the next day a brownish spotting again while wiping.. i was 5 days late with two negative HPT.. on march 21st after eating out, i had an instant reaction where i vomited and also had diarrhea.. i vomited twice and had diarrhea along with urinary infection throughout the night. I also had spotting and now there are chunks of blood clots. what could this mean?