A few thinks can be the cause of the gaining.
- Your fluid intake, is a bit much. 2 liters for a healthy person is enough, when you exercise you need more. (I have been surviving on 1-1,5 liters a day for years) Also make sure that 90% is water. no soda's and not too much coffee or tea
- Fluid retention, my doctor always said, you don't gain 2 pounds of fat overnight, but you can gain 4 pounds of fluids overnight. Take note of your sodium intake and your general electrolyte intake. Make sure that you are keeping sodium low. (for a healthy person it should be below the 6-8 grams a day, depending on how you feel with the lower salt. I am eating below the 2 grams for 10 years already and have minimum fluid retention.)
- Increase in muscle. I found out when starting primal last september that due to the increase in protein in my diet my muscles grew a bit, even without exercising. Like my body was healing them or regenerating them thanks to a more suitable amount of protein.
- your body is rebalancing, I only now just dropped 2 pounds since September. When I started PB, I gained a few pound, lost them again, gained them again, lost them again. I fluctuated between 77 kilo and 79 kilo. Now only after 4 months I'm at the 76 kilo... I realized that after years of eating junk and candy my body needed to fix all bad things first before it could focus on loosing weight and burning fat. It needed the healthy stuff to get everything in order first. I called it my re-balancing period.
I would advise you to be patient, it will take a while to loose weight. (I can't believe I'm saying this, I was just as impatient a month ago)... but as long as you keep the carbs between the 50-100 grams and don't eat too much calories you'll get there.
Pay attention to your calories, a lazy couch potato eating 300-calories even while keeping the carbs low will gain weight.
To speed up the weightloss, start introducing sprinting once a week. I found that it really worked.