Like Adgal said, it is unlikely that you are pregnant. At 43, you could be entering perimenopause, and again as adgal said, your hormones could be off balance. Irregular periods and lots of symptoms that mimic pregnancy seem to be very common when you enter your 40's. The results of your blood test will give you a definitive answer, and if you took a HPT and was negative, I'd bet the bloodtest is too. Best wishes to you!
It's highly unlikely that it's due to pregnancy (although anything is certainly possible). I don't think anyone here can tell you exactly what's wrong with you as there are so many reasons you can miss your period. There could be a hormonal imbalance of some sort, it could be early onset of menopause or even stress, extreme weight loss, the list goes on. If you had blood drawn, I assume you are seeing a Dr. I hope you find out what's going on, and I wish you luck!