Low blood pressure causing anger; augmentative mood and irrational logic?

My girlfriend has had low blood all her life. We don't argue that often, however when we do, often the discussion ends with her saying her blood pressure is low and she can't continue to talk. During the conversation, I often notice her logic breaks down, and she is offended by what I think are normal statements. She also can get quite angry for no apparent reason, relating to what we're talking about. I'm not after relationship advice here, I would like medical advice. What can I do to prevent this from happening, and how can I help her with her blood pressure in the future - for her well being. I am considering getting one of those blood pressure straps so we can occasionally monitor it. Other ideas?