I had something similar happen to me. I fluctuated up and down between 158 and 156 for three weeks. It would go up, and it would go down, but never under 156. After two weeks of that, I finally got down to 155. Still took another week for my weight loss to get back to what it was before then. Now I'm losing fairly steady again.

I went to the internet and looked up what could cause a stall on low carb, and I changed three out of the four that I read about. I lowered my artificial sweetener count (I had started drinking tea with Splenda), I lowered my protein count (I was blowing past my protein macro daily), and I stopped eating/drinking Atkins shakes/bars every day. I also read that dairy causes a stall in some people, but I couldn't give up cheese. I'm not sure if making those changes helped, or if I would have started losing weight again without them.

A lot of things can make your weight fluctuate temporarily, like too much salt. It sounds like this is a fairly new thing for you since you've only been doing this a month, and you had success the first 3 weeks. I would make sure that it's not just a temporary fluctuation before I panicked. Make sure you drink water.

Then I would look at the artificial sweetener and my macros. I also made sure that I wasn't miscounting my carbs. I was getting a little bit lazy about measuring things, so I went back to measuring everything.

I was hesitant to up my calories. Unfortunately I couldn't lower mine...I was all ready eating at around 1200 - 1300 (I don't exercise near as much as you, and I'm way too sedentary...trying to fix that). But you could up your calories and see what happens. The worst that could happen is you gain a pound and realize that it's not working. Weight loss isn't a race. Although I have a hard time remembering that myself. I don't have a lot of patience.

Good luck and don't give up! And hopefully someone will else will have some advice as well that's been doing this longer than me (I've been low carb since the end of February)

Some people have issues with caffeine and artificial sweeteners. You can't directly check for an insulin response to those easily, but you can quickly check if you have any rise in blood glucose (BG). If you don't have a BG tester go get one at your local pharmacy or online (they're cheap and so are the testing strips).

So here's the test:
1. Test your BG level
2. Drink a cup of black caffeinated coffee
3. Wait one hour
4. Test your BG again

If you see a significant rise in your BG then your body may not handle caffeine for this diet. If the caffeine is fine then repeat the test using your artificial sweetener of choice.

For me I've found that both caffeine and artificial sweeteners impact my BG. Ignore everything you'll find on the web about them not having any impact because I believe that you have to figure it out for yourself.

Personally I started back on March 1st. I lost about 16 lbs the first 3 weeks then stalled or gained for the next month & 1/2. I almost gave up but I felt better so I kept ketoing on. During this time I did very little exercise, my energy was rather low, so just daily walking. In the last month I've finally started losing again (today lost 2.5lbs) and I'm down 23lbs and have started strength training for the first time in my life. Stay the course. don't be afraid to eat more, especially calories from fat.