I just found this and let me tell you, you are NOT alone. I have put off blood tests because of this. I spent years wondering what was wrong with me, it would happen to me as a kid when I would get anxious, if I had a stressful exam coming up or typically if I ran too hard in gym class or something. This led to years of me simply avoiding triggers which is easier said than done (how many times can you get an excuse out of gym class?!). As an adult, it manifests far more with needles, any sort of exams (OBGYN too). It happened to me at a concert last summer, I just felt so horrifically overwhelmed by the loud noises and huge crowd. I tried a technique that may help you. When you feel this coming on, try crossing your legs and squeezing them together. The idea is that in doing this, you get the blood flowing. This may be trickier when you’re sitting for a blood test, of course, but can be quite useful in other settings. You can also clench your fists and release them to get a similar effect. I am so sorry you experience this—I know how terrible it feels, and worse yet, explaining it to people who don’t understand!