vision loss in right eye??

for about a year now, everyone once in a while my vision in my right eye will suddenly go black and patchy. idk what it is, it usually happens if I'm laying down for a while and then I sit up, or if I put pressure on my head for a few minutes. it goes away after a bit and also sometimes goes away if I flash my phone light at myself. it's starting to happen more often. I need to make an appointment for glasses anyways and am going to mention this when I do but does anyone know what it could be?!