How many calories are you eating each day? How many are you burning on your workouts? Could be two things IME: needing to up your calories a bit on days you workout or just giving it a couple of weeks for body to adjust to the new activity level.
"There's no such thing as failure. There's only feedback. Stop judging yourself & start recognizing it for what it is, an entry point for learning. The truth is, if you're not failing then you're not really reaching & really trying. If you don't like the outcome of a situation, learn from it, make corrections & reapproach wiser & stronger until you achieve your goals."