You should ask your doctor.
I've blacked out several times after getting allergy shots. It's always the third shot that does it. It not even that it hurts. I asked about it and it was some kind of common reaction that people have. Maybe it is something like that.
The first time I started having stress headaches when I was 18, I thought I had a brain tumor. The doctor told me to take some aspirin.
Maybe it's nothing but you should ask somebody who knows what they are talking about - not people here.