Have been losing approx 1lb per day. Yesterday ate 530 calories less than daily amount plus burned 240 calories but gained. 6lbs. What am I doing wrong? Ate no junk food, low fat, cals, carbs.

Not a daily effect. Keep a dietary journal and graph your weight on a weekly or bi-weekly basis. Add cross training: high and low impact aerobics and remember that diet= what you eat, what you do and who you are. Exercise with a picture of yourself at a thinner weight ( who you are ) and also have a picture of yourself ( or someone you want to look like) placed on your dressing mirror. High protein, 1500 cal diet.
Weigh once a week. Weigh yourself once a week, at the same time of day- preferably in the am. Preferably without any clothes. There are too many fluctuations and reasons for them to expect a weight loss after one day of activity. By the way- one pound weight loss per day is not healthy- too much too soon is not good. Consult a nutrition expert for help.
Maybe. Everybody can fluctuate depending on the calories they need and the water content. You may want to get fully examined so that the doctor can provide some proper nutritional advice and also check your blood work for possible thyroid issues electrolyte issues etc. Good luck.