Stopped taking Depo preva, having pregnancy syptoms, negativ

On November 25 was my last period, it lasted almost 2 1/2 months. I bled all the way through to January 10 and havent bled since. December 27 would have been the next time I was supposed to get the shot but I canceled it and now I am having signs of pregnancy. I took a test on March 3 but it came out negative, Could it have been too early to detect if I was pregnant at the time? Can I still be pregnant or is it because of the irregular bleeding??