What Causes Upward Surge And Pressure In Head With Feeling Of Passing Out?

I have just had what I can best describe as a surge upward and then a pressure from my head pushing downward. I felt like I was going to pass out and, of course, my heart was racing. I was at my computer when this happened so I went to lie down for a minute and then returned to the computer to try and check this out. I am 2 months short of 79 and in excellent health until recently when I have been experiencing cramps in my abdomen (4 days).