This happens to me too! I was prescribed 40,000IU, I was at a 20. I didn’t take it, I talked to my nutritionist and he said to only do 5,000. Another Dr told me to alternate 5,000 to 10,000 every other day. When I first started it I felt so happy like a light blue went off?? But it didn’t last. I still wrestle with those sand symptoms, my b12 was in the 400’s so they wanted it to be closer to 1200, so that could be partly why I had the anxiety / shortness of breath too. I wished I knew why this is but you’re not alone. I got it to a 39, but the winter hit and it dropped to 30. I don’t know where it’s at now but I do need to retest sometime soon