Yo-Yo-Ing

Well I'm annoyed with my body.
Friday was a feast day, so I feasted. Had about 1900 calories. We then made plans to go out on Saturday, so I decided to do a double feast day, as I needed soakage and the alcohol would ruin a fast day anyway. I intended to fast on Sunday but was tired and feeling eatey so I ate. Very badly I must say - we got a McDonald's AND a pizza. But, I wasn't too worried - figured it would do for my kick-start plan.
However, when I weighed myself on Monday morning, I had gone up FOUR kilos, or NINE pounds!! I freaked! I figured that if I fasted on Monday and drank loads of water, it would come down quickly again. I hoped that it was mainly water retention. So I fasted on Monday, yesterday I had a half feast day - I only ate 1000 calories - and today I'm fasting again. I weighed myself this morning though and I'm still about a kilo, or two pounds over the weight I was before the weekend.

I just don't understand it - I mean, I know I ate badly but still - four kilos in three days?? That seems excessive.
Now I find myself worrying that when I've finally reached my goal weight I'll just put weight back on really quickly if I'm eating about 1800 calories a day.
I'm annoyed. I wouldn't have minded if I'd gained a kilo after the weekend, but four just seems unreasonable.
Am I right to worry? I don't know. I wish I knew a doctor or nutritonist.