Glasshead has hit the nail on the head. The brain's ability to regulated blood pressure can get damaged. Your sensitivity to this is a strong reason to make every effort to avoid another head impact. Second Impact Syndrome is a result of an impact while this regulatory system is dysfunctional.

Here is a link to Buffalo concussion.buffalo.edu

I doubt another MRI is of any value. On home therapy you may try to exercise this system is simple. Sit in an upright chair. Slowly bend forward putting your head below your knees. As soon as you feel this fullness, slowly sit back up. If you do this on a regular frequency, you may be able to help your brain restore this function. You can vary the amount you lean forward to fit your comfort level.

The Buffalo protocol uses repeated increases in pulse rate and blood pressure to re-acclimate this system. Their exercise protocol establishes a limit of exertion that causes head ache and other symptoms. Then, you exercise to a BP and pulse that is 10% below this threshold. You increase the intensity by small increments about every two weeks or so. Slowly, the brain relearns how to moderate BP. It is not helpful at reducing the cognitive and memory symptoms of concussion.

Good luck with this as you carefully try it.

My best to you.