Doesn't sound like burn-in to me, sounds like the display might be outright failing or defective. Especially since the technology used in the 550 is not susceptible to burn-in typically. And especially since we're talking dead pixels. Is it under warranty? Because if it just happened, I would definitely try to get it repaired or exchanged.
Hmm, it sounds like something is up with your display.
Screen burn in is not possible for a LCD display such as the 550. Maybe your phone might have overheated or so, but this is still strange. I would suggest contacting a service center to get your phone looked at.