I Am Having Very Low Bp From Past 6 Years,

I am having very low bp from past 6 years, my age is now 25 in 2015. I used to faint in my university due to this, sometimes my brain freeze and eyes got black out. I am having heavy menstrual cycle of 5 days its painful but heavy after that bp falls to 80/40 and sometimes 90/40. Please help out for problem.