Pot and acid do have a tendency to interact. Try to think qualitatively about the hallucinations you experience when you toke (if you hallucinate when you toke). Is it the same, or different than the thoughts and emotions you experience when you're tripping? If you feel like you're tripping with the same intensity from toking as you do from tripping, then you might be allergic or something. Lol. Idk. As far as your neck I doubt it's related to the acid. Just go to your doctor. That's my advice.
LSD can cause vasoconstriction, which of course may be more significant for some people than others. But much of what you describe sounds like it might be Psychosomatic.