Hi Sarah! Welcome ;D
What you are describing is sooo similar to what I experienced and so did most of my female friends (although I think guys to some extent experience this too). Once you reach a certain age, mid-twenties to early-thirties our metabolisms slow way down. Thin as a child, skinny as a teen, lean as a young woman seemed so easy compared to nowadays.
The best way to naturally increase your metabolism is to get aerobic exercise regularly and make sure you consuming enough calories. Check out this information about increasing your metabolism www. weight-loss-center. net/speed_up_your_metablism. html (take the spaces out of the link)
Of course, if this has been a really sudden change in your metabolism you may want to make an appointment with your doctor just to make sure everything is OK.