Get it checked out by an optometrist. It sounds to me like it could be some sort of defect with the retina. Maybe a fluid sac or a tumour behind it. If left untreated you could lose your sight in the eye.
Of course it might not be anything that dramatic. I'm sure there are loads of conditions which I've never heard of which could be causing your symptoms. But as far as your eyes are concerned, it's never worth taking any risks, so always think about the worst case.
I've had a tumour in my left eye since about 2010. It's called a choroidal haemangioma and it's basically a sort of benign fluid tumour which causes the retina to bulge. The symptoms are varied and include dark shapes in the centre of the field of vision, greatly reduced contrast perception, reduced colour perception, and distorted vision. Just before I went in for treatment on it this summer, I made this mockup to try to illustrate what it does to my vision. The left half of the image represents how I see with my left eye, and the right half represents how I see with my (normal) right eye.
(Obviously that's not how things appear day to day. My right eye effectively takes over, so what I see over the whole field of view is quite good. But if I close my right eye, what I see is quite bad.)
The good news is that it's treatable. I had laser treatment in June after my eyesight in the left eye had deteriorated quite markedly (from 6/18 to 6/60). It's called photodynamic therapy and it targets tumour cells by infusing them with some sort of photosensitive chemical and then zapping them with the laser. It seems to have had a positive impact. I'm currently waiting for the 6-month review and I don't have any numbers yet, but most of the symptoms are significantly reduced. I might need a second burst with the laser next year, but it's really not invasive at all. If I hadn't had the treatment I dread to think how far the eye might have deteriorated, give how rapidly it was going downhill at the time.
Get it checked out.
