What Causes Fainting And Vomiting While Suffering From Constipation?

I am 85 female. for the last 2 0r 3 years, I have had several attacks where I suddenly feel faint, then pass out and vomit. I have a problem with constipation and usually it s when I m in the bathroom trying to have a BM. I donot feel nauseated until just before passing out. I am OK after that. YYYY@YYYY