After Bowl When I Stand Up Or During Toilet, I

After bowl when I stand up or during toilet, I feel lightheadness sometimes. Sometimes I feel warm head and dizziness or blank head. My ECG, bp, lipids, approx everything is normal. It happens with me since my childhood. It is problematic when I try to study at table because irritating and head pain and slight lightheadness. Will you please or any one of you help me. I have seen to many doctors they just give me anxiety Medicine and vitamin. But still I am facing same problem time to time not always.