The healthy body weight of a female 5’6″ is 130 pounds. An appropriate weight range is 10 percent above and below that or 117 to 143 pounds. You are definitely not overweight, especially not 15 pounds. Because of the daily swim practice you get, you probably have more muscle than fat. Muscle weighs more than fat.
I would suggest you talk to your coach about your weight and that you feel good at 123 pounds. Getting down below 123 pounds could probably seriously decrease your swimming performance by decreasing your muscle size. A 15-pound weight loss would also deplete your glycogen stores needed to compete.
I would like to suggest a good, basic sports nutrition book. “The Athletes Kitchen” by Nancy Clark. It contains good information as well as some recipes. Also, the National Dairy Council in Rosemont Illinois has a pamphlet “Food Power, A Coaches Guide To Improving Athletic Performance”. Both are specifically geared to the athlete and performance. Buy one for your coach!