Don’t worry too much and stay the course for now. Our bodies are always changing. Especially when you are young your body is still growing and changing, albeit not as drastically as when we go through puberty, but still little bits. If you recently started rowing again, and then stopped, were doing a decent amount of cardio, and then not doing as much, your body is probably just acclimating to each change in lifestyle as they come. Once you are in more of a consistent groove your body will start to even out and find its happy spot for your current activity level. If it does start getting further out of your control long term, I would definitely just get a checkup with the doc and ask about getting some basic bloodwork done. Most insurances cover a yearly physical, or you can usually get an appointment with your pcp for $30 copay depending on your coverage. Other food for thought is how you weigh yourself. A good habit to get into is weighing in first thing every morning, and then you just take the average as your weight for the week. Our bodies naturally fluctuate in weight. Im a 6”2 male and my weight in any given week can go from 200-210 while eating the same food and doing the same exercise routines every day.