When should I get a blood test?

I am 4 days late for my period and still no signs of AF coming except pressure in my lower stomach and period cramps. My periods are never late and the one time it was late I was when I was pregnant with my first which was two years ago. All my pregnancy test are coming back negative...I have no pregnancy signs or signs of AF/spotting. I've been feeling extremely wet below but everytime I check its nothin but cm. Could I still be pregnant even though I'm getting bfn? Should and when should I go for a blood test?