Is it possible to be pregnant after a negative blood test?

Went to the E.R with pain, come to find out I have a UTI. They did a urine pregnancy test and told me I was pregnant. I said are you sure I just had my son 2 months ago. They checked it again and it was still positive. I came back the next day for a blood test and it came back negative. I am bleeding like my period so I didn't think I would be pregnant but I am nauseous in the mornings like I was with my son and have been for little less than a week. Help please! I'm having an emotional Rollercoaster ride!