I have had my struggles with water retention a couple of years ago. The issues were most likely due to hormonal imbalance. The crucial factor being thyroid health. I will not go into details, but the issues became medically relevant. In the meantime I resolved those medical issues and I lost a bunch of weight on low carb. First on Medifast, where PMS and water retention basically disappeared. When I hit a plateau last winter, I switched to much lower carbs using IP. This worked great for weight loss for a couple of weeks. But the very low carb eating also exposed what the commercial does not tell you. Namely, carbs are necessary for the production of certain hormones, including estrogen/progesterone, and active T3 thyroid. When you are sublinical hypothyroid, these facts may well be exposed as some above also experienced. I personally noticed it when I started to experience the exact same symptoms I hoped I never revisit. One of which is water retention. Though, thankfully, I noticed the symptoms early and stopped low carbing.
In your case, I would get a complete thyroid panel and also have your sex hormone levels checked. Just to exclude a situation that can cause long-term negative effects on your health if not corrected. Otherwise, keeping a good salt balance in the heat may be hard to do on low carb. You actually need more salt than would appear healthy, at least in the beginning. A good source of scientific knowledge is Phinney who published two books recently (low carb living and low carb performance).
Otherwise, when in doubt, take a diet break. I am actually slowly regaining my thyroid hormones and am still loosing weight, though much more slowly (but I am also eating significantly more now).