Spot on the sensor of my NEX 7 :(

Since yesterday afternoon I got a funny spot on my sensor. In the morning pictures were fine, and three hours later, after changing lens home, the spot appeared. On the sensor it looks white, of about 2x1 milimeters. On the photo it looks brownish. I don't think it's possible that anything came in the camera changing lenses home, which makes me suspect that the sensor developed this spot. Is this possible?

I bought my NEX 7 2 weeks ago. I don't know if I should send it to Sony under guarantee (slow but for free) or have the camera serviced privately. Advise?

Enrique