What Causes Delayed Periods And Negative Pregnancy Test?

I AM 11 DAYS LATE AND MY PERIOD HAS ALWAYS BEEN ON TIME. I HAD INTERCOURSE AFTER MY PERIOD AND ON MY MOST FERTILE DAYS. I HAVE BEEN HAVING PERIOD LIKE CRAMPS EVERY SO OFTEN AND MY HEAD AND BACK HAS BEEN ACHING I GET REALLY DIZZY FROM TIME TO TIME AND I HAVE NAUSEA. I HAVE TAKEN 4 HOME PREGNANCY TESTS AND THEY HAVE ALL CAME BACK NEGATIVE IS THERE A CHANCE I CAN STILL BE PREGNANT?