Am i replacing fat with muscle? am i doing something wrong?

i am in the process of getting ready for Air Force basic training and im trying to lose my remaining 28lbs so i can be accepted. when i9 started i was losing 4lbs on adverage a week i even lost 7lb in one week! but then i started working out with weights and doing crunches and what not before i added weights i was just walking and it helped me but i never can find the time after work bec its super dark. anyways to the main point. ever since i started working out now im only losing 1 lbs a week or not even a pound and im not gaining so whats going on? am i turning fat into muscle? or is it something els?