Probably just water weight! I'd say weigh again in a few days and see what it says.
And avoid thinking about eating some nice things sometimes as cheating. You weren't binging and that's the main thing. And especially if you weren't eating much other stuff! It's good to enjoy yourself and have a little slice of cake.. just not the whole cake.
Weight loss isn't always so clear and it certainly doesn't follow our exact calendar. Some weeks I gain or only lose half a pound or less even though I was doing everything right. The math just doesn't add up. Buuuut, in a week or two I might suddenly lose 3lbs and this is sometimes overnight.
Don't beat yourself up about it, just move on
Great job on losing 36lbs so far, that's awesome!