Hi As there is sudden gain in weight and no obvious change in your diet then we need to find out the cause. Need to rule out common causes like hypothyroidism and PCOD ( if your monthly cycles are irregular) which you need to get a blood test and USG done. Also irregular meals and inadequate sleep on long run can cause weight gain. So a proper diet which suits your lifestyle and improving sleep will help you immediately to get a hold over it. Along with this some form of physical activity is must like walking or any form.of sport which you enjoy or may be a dance class. You haven't mention about your water intake. Drinking right amount of water at right time will also help to keep a check on the weight.