Braces off with spots?

I just got my braces off after years a few days ago and ive just noticed on a couple of teeth that had the metal donuts on that there are brown spots on the inside. the man taking them off said they were the hardest to get off he's ever had. The teeth aren't sore or sensitive at all. they aren't holes either, they just seem to be there and it worries me. What are these spots? they aren't very big, or horribly dark brown. just light brown spots that won't go with brushing.