Hi, I Have Been Lightly Spotting For 2 Weeks Now,

Hi, I have been lightly spotting for 2 weeks now, the pad is brown but when I wipe its red. My Brett's feel heavy and look big and are tender to touch. About a week ago something brown and worm like came out and it smelled foul, but not too much blood on it. I took a pregnancy test yesterday and it was negative. I have continued to take my birth control pill as normal through this whole process. I'm concerned what this is