I recently returned to masters swimming in June of this year, after more than a decade lay-off. I just turned 73, am 5ft. 5 in. tall and weigh 120 pounds (female). My club swims in an outdoor pool early in the morning, beginning at 5:30 am, when the water in the fall/winter is around 78-79 degrees. I have noticed since the weather turned cooler that I can swim around 2500 yards, and then all of a sudden I start shivering, and even if I try to keep swimming, I can't warm back up. All the shivering leaves me tired and sluggish after this relatively short workout. I try to maintain a healthy diet, mostly vegetarian and am wondering if I need to try to gain weight, start protein drinks, hit the weights hard, etc. I would appreciate some feedback on this. I need to do something so I can make it through the winter in the pool as well as increase my yardage. Thanks!