Pregnant or just false positive results?

Ok I have taken 4 hpt an they have all came back positive some lighter than others...but I got a blood test done at work today an it was negative what are the chances of the blood test being wrong an me having 4 positives test results im so confused an it dont help the fact that im having alot of left side lower abd pain an fullness in my pelvic area just dont know whether to be happy im pregnant or whats going on...has this every happend to you or anyone you know all information will be helpful thanks