Depression-related possible seizure...any opinions welcome

Hello, I have been diagnosed with depression and came off long-term Zoloft usage about last April. In October I began occasionally experiencing a very strange feeling which I came to suspect might be some kind of seizure, but my doctor pooh-poohed that idea. First I get a familiar feeling of depression, but the strange thing is then I start to feel a little like I'm floating, and there is a distinct "whooshing" sensation down the back of my head, my back, and legs, as though the blood was rushing downward through my body. My eyes often go a little bit out of focus and I feel as though I am "drifting" slightly physically. If I start moving my head around and thinking about something right when I feel this coming on I can often avoid it; it seems to happen most if I am just sitting still, not focusing on anything. Generally it is accompanied by a wave of rather severe depression which lasts longer than the physical sensations. My heart rate and everything else remains normal. At no time do I lose the ability to move, see, or hear, although it is very distracting and I can't concentrate on what people are saying when it's happening. I would greatly appreciate any insight as to whether this sounds like any seizure-like symptoms anyone has experienced. I'm not sure if that is a totally off-base idea or if I should take it to a different doctor who might follow up on investigating that possibility.