I have been eating healthy and increasing exercise but still gained weight, 15 lbs in 2 weeks. Could I be pregnant and what are the signs?

You could. It is possible that you are pregnant but there are other causes of weight gain as well, like thyroid disease. Other pregnancy signs are fatigue, breast soreness, nausea and not getting your period. Either way, you should see your doctor about that much weight gain.
Water weight. To lose 1 pound you have to eliminate 3500 calories from your diet. To gain 1 lb you add 3500 calories. Average adult diet is 1800-2400 calories/day depending on age & gender. So to gain a pound a day you must be eating 2-3 times your normal diet. Unlikely. This is more likely water weight due to retained salts. Have your doctor check kidneys & heart right away.