What Is Long White Painless Bump On Back?

Hello,
Two weeks ago I noticed a long, white bump under my my skin on my back near my spine. It's about 1.5 inches long and protrudes out a little bit. At first I thought it was a bug bite, but it didn't heal. After two weeks, it's still there. It doesn't hurt at all and doesn't move when I rub it. When I arch my back, the bump disappears. When I erect my spine, the white bump becomes very conspicuous. But I have no idea what it is. Can you please help?