What Causes Palpitations When Lying On The Back?

I occasionally have palpitations when I am lying on my back. I feel slightly short of breath while it is happening. Usually lasts for 10 seconds or less and seems to resolve more quickly if I sit up. I have never felt any associated dizziness or chest pain. I take 50 mg of Losartan daily for HTN and 325 mg ASA daily because I have to remain seated for most of my 12 hour shifts. I have a family hx of CVA, HTN, and lung CA. I have NKDA. I have had labs (cbc, tsa, cmp) drawn 2 months ago and were all within normal limits. I feel as though I have an anxiety disorder that has been as of yet, undiagnosed. When I begin feeling anxious, I feel as if my throat is swelling.