newbie help!

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Hi there! I have 20kg to lose. For past 3 weeks I've been doing 1200 calories a day, and I'm also in final weeks of couch to 5k so running alternate days (where I eat extra calories)
Firstly, this week I've put on 700g, not much but psychologically very disheartening. Is this muscle from exercising? Also yesterday I was 100 under on calories, can I go 100 over today? Had a latte when out with parents :-(

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