confused, please help!

hello, i am a 5'2 female who weighs at about 115. i been on a caloric deficit and went from 120 to 115 in a week, with no exercise, eating about 600 calories a day. the week after i decided to add some exercising, which includes doing a 12 3 30 on the treadmill everyday. everyday since then my weight has not gone down and stayed the same at 115. there has been no changes to my diet, still eating at 600 calories a day. does anyone know why this is happening? is it because i started exercising when i usually dont? please help me out, im feeling discouraged and lost. thank you so much!