Can you not go back in to your orthodontist? I'm assuming your treatment isn't finished yet if you still need retainers, so if my one was anything to go by, you could just ring up and book yourself in for an appointment. Whether this is actual damage to the enamel or just normal after removing braces, you deserve an explanation and some reassurance, so get on the phone to your orthodontist and explain the situation. Also, are you sure about what routine you should be following now with regards to correct type of mouthwash to use, etc etc?
If these really are hairline fractures, enquire about them because the last thing you want is damage to the nerve or infection to set in.