You started spotting brownish stuff 7 days pass your expected period could you be pregnant?

Just to give some more info about my question...we have been trying to conceive for some time now and I took a pregnancy test a week ago and it was negative but I'm not sure if the test was done properly. My period normally comes on the 15th of the month; however, I have been a week or so late before. It is now the 23rd, and on the 20th of this month, I started spotting this brownish stuff, the same kind of stuff that normally occurs before each of my periods, it would last no more than 2 days. The only difference this month is that I have been spotting this brownish stuff with the occassioanl red blood...it's been 4 days now. It barely gets on a thin pantyliner...I was extremely moody last week, my breast are tender and they are normally tender but as soon as I would start to spot, the tenderness goes away, my breast are still quite tender today. I hope that we are pregnant but could it also just be a very late period?