Gaining weight

It can happen as you start working out more seriously, give it a couple of weeks and increase your water consumption.  Your measurements will likely change as muscle is more dense than fat and if you feel yourself getting bigger and your goal was to feel leaner then you need to have a chat with your trainer so you tone instead of build muscle.  Lighter weight / more reps.  Yoga as opposed to running maybe....

Keep it up you and your body will find the balance.