3 Fat Chicks on a Diet Weight Loss Community

I am having some radical weight swings. I guess 'radical' might be a strong term, but they feel pretty radical to me. Last week I was having a rough time staying on diet and I was gaining around a pound every day until I got back up to 193-194, at which point I got totally disgusted and stopped weighing completely. Then I went out of town over the weekend. Yesterday morning, back at home, I weighed myself and was 187, to my surprise since I didn't do much over the weekend to lose. Then I ate maybe 300 calories over my 1300 calorie goal yesterday and had gained a pound this morning, putting me in a thundering bad mood all day.
I'm really just having a hard time getting motivated again. I fell into the trap of relying largely on 100 calorie packs of cookies and Lean Cuisine meals and Diet Cokes, which I know sets me up straight away for a binge because that's all empty empty empty and full of sugar. Then my grandmother went to the store today and got three more boxes of 100 calorie cookies, "low cal" cereal, "light" potato chips, basically a mountain of carbs. I'd made out a grocery list with lots of protein and veggies and good healthy stuff but i didn't wake up early enough to give it to her before she went to the store. I felt like exploding (remember, i was already in a towering rage over that pound gained) and complaining about carbs and basically being ungrateful and nasty, but I didn't take it out on my granny.

Is there anything I can do to level out my weight or are these fluctuations natural? I feel like if I even eat all 1300 calories in a day I risk gaining weight, and sometimes I do. Why??