Are There Chances Of Pregnancy Inspite Of Spotting?

My boyfriend and I have been trying to concieve. My last depo shot was the first wk of Dec. I had a period the second wk of March and then nothing until the end of may when I spotted for 2 days. For the last two days I've been noticing some bright red blood but only when I wipe. Had a blood test done on thursday but was negative. I am confused I've googled it and it says there is still a chance of pregnancy. Can you please help?