5'4", 140lbs... Help!

I'm felling pretty self-conscience about my weight. I'm 33 and the largest of all my friends. I work out 3 days a week for 1/2 hour and eat a very healthy diet (lots of veggies, very little carbs). My weight doesn't fluctuate... Ever. I just ordered a bridesmaid dress in a size 8. How do I loose weight???