The Odd 'Bad' Day

Hi everyone,
I am new to MFP and really only using it to keep an eye on what I eat to maintain my weight. I recently gained a stone due to my starting early menopause (Im only 34 ), it was a horrible thing to find out, I have all the rubbish symptoms and I was gaining weight with no good reason, but taking HRT has balanced me back out and over the last year I am back down to my fighting weight - I am 5ft 3, 118lbs and quite happy like that, I have always been fit and worked out and love to run. I take part in half marathons when I can to give me a goal.

Now the background is out of the way - my worry is letting myself have a the odd day 'off' if you like, I don't know if it comes from being out of control of my body due to the menopause - I am infertile as a result and I don't have children and there is nothing I can do to stop my body changing in this way - so the feeling I get from controlling my food and exercise is becoming a bit of an obsession - I did once get very underweight when going through a bad period in my life and it was all about control then too. This weekend I went out for a friends birthday - had a great time, laughed, danced...drank way too much gin and ended up scoffiing rubbish food (as you do) I didn't even go THAT much over my calories but now I am freaking out that the weight will catch up with me - I was fine yesterday, normal day of eating, didn't exercise as I do have at least 2 days of rest and today I know I wont go over and I am going to the gym and for a run tonight....SO my rather round-the-houses question is...is ONE Saturday night really going to cause me to put on weight? I feel I know the answer, I just need some confirmation Thanks