Sudden ache in the head, left back brain, just a few seconds. what is it? and the cause?

Here's what it ISN'T: It ISN'T your brain. Your brain doesn't "hurt." The brain is completely insensitive to pain. The brain senses pain only in other organs & tissues, not itself; and you feel those pains in those other organs & tissues. Your post is vague: Did this happen only once, or does it happen occasionally, frequently, or constantly, and when was it or did it start? Your post as is cannot be answered.