Gaining weight despite diet and counting calories?

Patient: I am a 5 foot 7 inch young female. About 2 years ago I was at my heaviest at 175 lbs. I had lost 40 pounds on my own and was down to a steady 138 lbs. My lowest was 133 lbs. Now in the past few months I have have gained 10 pounds back. I am now at 147 to 148 lbs. Ironically I maintained the 45 pound weight loss for about a year with no tracking of my diet and no regular exercise or exercise plan whatsoever. Now that I track calories and nutrition I am shocked. I must have been eating 2500 or more calories a day or that year. After that I began to gain weight which totaled 10 pounds. I began counting calories, I eat 1200 to 1500 a day. My net calories is often a loss of 800 to 1000 per day. I do find that I am often low in potassium intake. I have a hectic schedule and therefore am not able to exercise regularly at this time however I do walk and stand often throughout the day. I usually do not find time for breakfast. Just in this past week I gained 4 pounds while I was sticking to my calorie diet. I think some stress caused by a new work situation may be to blame for the weight loss but I cannot believe that I am gaining and gaining when I want to and think I should be losing and losing. I am really at a loss for what to do. I would greatly appreciate any advice etc. Any foods that might help? A recommended exercise plan? Healthy breakfast ideas? Ways to combat stress weight gain? Thank you SO much.