I have this problem as well - since being prescribed Tamsulosin for BPH. In my case its Postural Hypotension (I think)
http://www.nhs.uk/Conditions/Blood-pressure-(low)/Pages/Symptoms.aspx
A **** nuisance. It does cause a problem if you are learning to fly!
It particularly happens after exercise for some reason, and in warmer weather - I "grey out" momentarily. I think its because the Tamsulosin affects a muscle in the groin somewhere that contracts when you stand-up, the muscles purpose is to maintain blood pressure so it doesn't drop momentarily. Tamsulosin and related medication relaxes this muscle so it doesn't work properly otr something.
I did have the problem when I first was diagnosed as T2, the BP tablet I was prescribed initially gave me the same symptoms, the GP swopped it for Irbesartan and that cured the problem until I needed the Tamsulosin later. I was swapped onto Tamsulosin to try and get rid of the symptom and it helped - but I can't remember what I was swopped from.
Obviously the cause in my case might not be the same as in your case. I did get a BP monitor to see if I could identify what's happening, but of course I found that by the time I could measure my BP it was all back to normal!