Why can't I lose weight when I've been busting my butt?!?!?

I'm a 36 yr old female, 5'3, 169lbs. I have been eating low carbs (my carbs come from fruit, vegetables, and some low carb pitas) and doing zumba 6 days a week for almost 7 weeks now. I've lost 11lbs, but for some strange reason NO inches. About 5 yrs ago I did low carb (Atkins) and lost 50lbs in 3 months without any exercise at all! Over the years I was always able to fall back into low carb to lose winter weight. I'm becoming very frustrated and discouraged. I have not had more than 20-30 carbs in one day, and probably eat about 1200-1500 calories. I drink about 70 ounces of water, I don't consume any caffeine at all, and I take fish oil, and vitamin B complex. Is there anyone out there that could have a suggestion for me? I know your only supposed to lose 2lbs a week, but like I said, I've been doing this on and off for 5 yrs and usually atleast lose 20lbs in the first 3 weeks. I know I'm getting older, but the Zumba kicks my a$$, so you'd think the fat would be melting away! Any one with advice would be great! Thanks so much :)