These things happen sometimes. Could be 101 reasons. Starting exercising more is fabulous, make sure you're drinking plenty. I am sure you will have a big loss next week. Just a quick thought, when you say you stuck to plan, have you been having all 15 syns a day? A lot of people make the mistake of trying to reduce them when they want to speed up their losses, but it has the opposite effect, because our bodies go: "arh whats happening?! I'm so confused" and act like 3 year olds and have a tantrum!
Deep breath, relax, tomorrow is another day, and the start of another sw week.
Hugs,
Natt xxx