If you haven't read Ultrametaolism and Untraprevention yet, go out and get them. You can find out more about the author, Dr. Mark Hyman, here:
http://coaches.aol.com/wellness/mark-hyman/main
Your metabolism does change in your 30s, but there is a lot you can do to beat it. definitely change your diet to one featuring whole foods and low in processed foods, make sure you are getting enough sleep, but above all, exercise and get fresh air. I know when you are tired and busy, you feel like the last thing you have time for is exercise, but really, take a ten-minute walk on your lunchbreak, or put on your running shoes and take a 15-minute jog around your neighborhood when you get home. It will energize you when you are awake, and help you get a better night's sleep so you feel rested.
Good luck!