You are a perfect exception to the body fat norm. You are thin with a low body fat. Use the calculator without inputting your body fat to calculate a healthy body weight, which should be 144 – 176 pounds. Ignore your BMI calculation as your actual 8% body fat may be much lower.
Yes you can weigh 145 pounds and still be 8% body fat. No, you will not be overweight. Try your same data including body fat but change your weight to 145 pounds. It will calculate a healthy weight range above and below 145 pounds.
To gain weight that is more muscle and less fat, weight lift to maintain your low body fat and increase your muscle mass while gaining weight. You can choose to gain 1 or 2 pounds per week and the calculator will determine how many additional calories you need to eat. Leave the fat at 30% of calories as fat is a concentrated source of calories. To gain weight, you will need to eat a lot more food than you usually eat as carbohydrates or proteins are move bulky than fat.
I would be surprised if your elbow breadth is 3 1/2″, try measuring your elbow again. This time, place a ruler on a flat surface in front of you while placing your left fingers on the protruding bones of your right elbow, lay your elbow on the ruler. Then remove your elbow but leave your left-hand fingers on the ruler. Read the number of inches between your fingers to get a better measurement. Then re-run the calculator.