I have noticed that my stomach growls loudly anytime I lie down on my back, and only when I lay on my back. I don't notice it when I lie on my side. Lately the noise comes on within ten minutes of attaining the supine position. I had been having some bowel sounds for some years but it seems to be louder in the last six months, and now it even wakes me up. I don't have any other symptoms, illness or medication. What is going on?