Talk to Internal Medicine on Dizziness And Pressure In Head

i am having this issue from 3 months on and off like with the gap of 10 to 15 days that i feel alot of pressure in my head and nose. My blood pressure is always low. whenever i go for test. My HB report is fine . Now from 1 week I when i lay down i change my side or i get up and sit i feel very bad dizziness. I happens all the time even when i move my head. and also i feel pressure in my head. Also i am having Gas problem from a week. What could be the possible reason? i visited doc 3 weeks ago he said all your tests are normal and gave me Surbex z. but its not helping at all.