What Causes Vertigo On Lying Down?

i am experiencing mild veritgo mostly when lying flat in bed, mild shift right then left. Hx. of sinusitis and inflamation with great nasal passage clearing with Flonase. But have had troubles with feeling fullness in my head, right ear and pulsing sound i can hear in my right ear non stop for 4 months now. My BP runs 112/70 s - 127/80 s. I exercise on elliptical or jogging at least 30-40 min 3-5 days a week without any problems. I have had my eyes, sinuses and ears checked and they are fine with no fluid behind ear drums. My hearing test results was exceptional. I did have 4 episodes of BPPV over a year span (with last 5 months ago) in which i saw 2 physicians and also a ENT. All agreed it was the BPPV and was treated by PT for manuevers and have done preventively daily for months with good results. So the vertigo I experience now is mild compared. The recommendation now is a MRI and MRA of my brain. What could this be?