Lots of things can cause your weight to fluctuate: retaining water, that time of the month, measuring in a different spot, and/or illness, among other factors.
It should go back down; and the scale is not really your friend if you know what I mean. It shows the fluctuations in your weight really well. Your weight can fluctuate several times in a day, week, etc. Did you weigh first thing in the morning after urinating? I've read that's the best time. Also, I would only weigh once a week (although I have to peek! more frequently than that).
You are right: you will gain muscle mass, but I highly doubt it happened that fast that much.
I wouldn't worry about it that much. Be patient and try not to let it worry you overmuch. It's just a small bump in the road. Don't panic, we all see fluctuations in our weight. I noticed that some of my measurements have remained the same and increased, in some cases, but it then goes down.
I just keep telling myself that I'm on this journey for life, and that does help me. God's blessings!