You should not worry about it. If you are doing well eating at noon and 5pm and you have weight to lose, your body is actually consuming more calories than just what you intake. You won't waste away on a short daily fast like that. It's actually a more normal pattern of eating.
If you want to stop the weight loss or if you simply wish to consume more calories, eat more at your meals. More protein is a good idea because then your body doesn't have to consume its own protein and your body can use protein to create glucose and gain weight if needed.
Female, 5'3", 48, Starting weight: 163lbs. Current weight: 135.
Starting squat: 45lbs. Current squat: 145 x 4.