Hi there and thank you for the question. I am a Registered Nurse and will be happy to help you today.
Fortunately, you would not have gained 'true weight' this fast. With you being on your period, this is most likely water weight gain. A fluctuation of 3-6 pounds during this time is very common. Try to go back on your diet and watch your sodium intake (watch hidden salt in frozen meals, sandwich meats, soups etc). Also, you can squeeze a fresh lemon into 8 oz water and the lemon will help you shed some water weight.