Can I Be Pregnant?

ok, i have always had an irregular menstrul since i began my cycle at 16. Me and my boyfriend have been trying to have children, but it just seems like no luck. In dec. 2008 my mensturl came on the 17th, then in Jan. 2009 it came on the 7th, but when it came on it only spotted for 3 days then went off for two, then spotted for 2 more days. Then went off. It has never done that before, and even in the past when it has spotted, it usually come on fully within the next day. I have been feeling weird lately but im not sure if its just my concious. I know i have been experiencing heart burn, and stomach aches, and i have a weird eating habit, and sometimes feel nauseus. I have taken a pregnancy test in January and there was a negative response, then i took one today. Oh yeah, i havent seen my mensturl yet since last month. The one i took today said negative too. Whats going on? Ok heres an update with my situation....well for the past week or so i have been having pains in my stomach, sort of like cramps, so i was looking forward to seeing my mensturl but no sign. About three or four days ago i went to the bathroom to wipe my self and there was dark brown stuff on the tissue. i looked in my underwear and i didnt see anything, this happened for at least three days then i didnt see anything. I went to google and yahoo to see what did it mean, and the responses was it may be old blood from my last period. But i havent had a period since January. It didnt come on in feburary, and now its march i need help!!! ?ª