H.M.
Just a follow up - DO NOT DO INVISALIGN - it really doesn't work that well - especially if you have crowding. Trust me - $3000 later my teeth still aren't great (even wearing them every night) so please get another opinion or two before shelling out the money.
Umm - this has happened to me not once but twice - I had braces in high school and then did a full course of Invisalign in 2006 - and yep - my teeth have moved again - despite wearing the retainer.
Unless you want the big expensive of doing braces again I would ask to be fitted with a retainer to "hold" them in their current position. Or you can order a retainer mold on line (I got one through Amazon) do your own mold at home and have a retainer shipped to you for about $80.
Good luck.