Low blood pressure can continue throughout the fast, there's not much you can do about it. There is no danger except if you actually faint and hurt yourself on the way down. The best thing to do is just to sit or lie down when you feel faint and rest till the feeling passes. Then get up slowly! It shouldn't effect your concentration so much though, that was probably a separate issue. I had low blood pressure every day of my 30 day fast, it didn't stop me balancing on my motorbike each day going to work or concentrating- if anything I was concentrating much better than usual.