Late period, bfn

I'm on the 40th day of my cycle now, and no signs of my period. I've been ttc for 4 months, and I have always been perfectly regular, have never been late. My cycle is about 30 days long on average. Yesterday I took a blood test and it came back negative. Being this late, I've read that if I'm pregnant it should be showing up on a test by now. What the heck is going on?