Skin Pigmentation Problem

My stomach area has developed a very noticeable pigmentation problem over the last 3 years, which seems to be getting progressively worse; there are patches of darker skin surrouned by much lighter skin, in a cloudy-shape like pattern.  I can no longer tell if there are dark patches on top of my normal lighter skin, or if the dark patches are my normal skin and there are huge blotches of much lighter skin everywhere on my stomach. The rest of my body seems to match the lighter skin, but I'm really not sure which is which.  This past summer I noticed the blotchiness spread around the side of my stomach to a bit of my back, and noticed the white parts did not tan in the sun, and as the dark skin got darker, I was very disturbed as the difference in pigmentation became extremely noticable.  

I have visited a dermatologist who at first thought it was fungus. After treating the area for fungus, no change was seen and he determined that he no longer believed it was fungus. I am now on a 2-month anti-biotic pill for bacteria as he now believes the pigmentation issue may be related to bacteria. I have taken the anti-biotics for a month and have seen no progress yet.

Just wondering if anyone else has experienced this strange loss of pigmentation on the trunk area and what it might be?