sounds like a strong vasovagal response maybe combined with a little bit dehydration/electrolyte disturbance maybe combined with the need to make your muscles not be tense. Do you stretch long enough before you exercise and also after you exercise? That is really important. The muscles in your neck are really important and you should do neck stretches every day, multiple times per day, along with isometrics. Keep up the exercise, that is good for you. Then anxiety did not cause this, it is a normal body function/reaction what you are experiencing. The anxiety started once you realized it and didn't understand why it happened, so now you are waiting for these weird sensations to happens and you are looking for them to happen. That's normal, too. I would suggest make sure you have a lot of carbs before you work out. After the workout have protein. Keep yourself hydrated. Wait a few minutes when you go from sitting to standing or laying down position, give your body a chance for the blood to flow through to everywhere so you don't get lightheaded and see floaters.