Hi Claire, I had exactly the same problem a couple of weeks ago.
After six non-cheat weeks I felt compelled to eat the biscuits my hairdresser had kindly provided with the complimentary cup of tea (the treat was supposed to be the haircut). I was cold, tired and exhausted - all factors that make me eat. After the biscuits, I felt more hungry and ended up snacking throughout the rest of the week because I was no longer in ketosis. The result was a 1lb weight gain.
I thought it'd be easy to re-start and ended up spending a further week of snacking trying to get back on-track. I'd be fine until 2-3pm then for some reason go and raid the food cupboard. I wasn't stuffing my face, nor eating the food in as large a quantity as I would have done pre-CD but, I'd justify the cheat by thinking that I was simply going up a plan (or five) to kickstart my weightloss back into action. This sounds ridiculous now that I'm typing it. I even had a couple of small, two-course meals.
Cake, chocs, crisps, biscuits, potatoes, gravy, lamb, vegetables, apple pie, crumble topping, ... these are just some of the foods I ate during those two weeks. Instead of a cumulative 1lb loss, I could have seen an 8lb loss for the fortnight. That's half a stone difference.
I tried again for another week to re-start, this time I lost 3lb. My heart wasn't in it to get back on-track properly because we had been invited over by the in-laws for a meal Saturday night and I knew I'd have to eat something or be subjected to scrutiny (I've not told them I'm following CD). I had roast chicken, berry crumble with ice cream followed by several cups of milky tea/coffee and a portion of apple pie with cream. I didn't want the pie, I promised MIL I'd try it and she went on for a couple of hours about it before I'd agree to a slice. She wouldn't let me cut a piece for myself and I ended up with a wedge the size of a small dinner plate. She is dieting herself and I've a funny feeling she's noticed my weightloss so was trying to sabotage things but, maybe I'm being paranoid. I was so stuffed, I haven't felt that uncomfortable since I was pregnant, I could practically feel my stomach pressing against my diaphragm. Oh yes, I also had a couple of small glasses of white wine.
I weighed myself yesterday, I'd gained four pounds so I've been taking a fibre supplement with my water to help things along before my next official weigh-in. This time I'm determined to succeed and managed to re-start properly yesterday.
When I adjusted my tracker, I realised I've only got another 19lb until I reach target, if I put my mind to it I can do this in 5-6 weeks.
My advice to you is, think about the weight you could have lost if you hadn't have cheated (i.e. 4lb pw) rather than the weight you actually lose/gain when cheating. I could have lost 12lb over the past weeks, I actually lost 4lb. A loss is a loss but, I could've done a whole lot better.
Hope you get back on-track soon hon. Good luck.x