What Causes Bleeding With Pill And Not Due For Periods?

Hi, I've been having blood spotting on my underwear for seven days now. My period is due this Tuesday/Wednesday. I am on birth control and missed one pill this month (took it a day late) and also took a pill later in the day than I usually do. I am sexually active but my boyfriend and I use condoms, and the one time that he didn't, he pulled out a long time before he could ejaculate inside of me. I'm a little concerned about the bleeding since I'm not sure whether it's because I took my pills incorrectly or if I'm pregnant. I've taken a pregnancy test a day ago and it was negative but it might be too early to tell. Help?