What Causes Vomiting And Headache After Receiving A Flu Shot?

I had a flu shot yesterday afternoon at 2;30 pm (double dose since Im 73 years old). About 8 pm in the middle of dinner I blacked out with no warning, for about a couple of minutes according to my husband, slumping over my plate. I then started throwing up violently, and shortly after got diarrhea. I then sat on the sofa while the debate was going on without really following it. When I got up at 11 to go to our bedroom I started shaking violently. this continued for nearly an hour, during which time I called my primary care physician. She thought I should go to the ER which is several miles from here. We decided to wait until morning. I slept for about 3 hours. Now (7:15) I still feel shaky with a slight headache. Could this be an allergic reaction to the flu shot? Can t decide if I should go to the ER.