Congrats on the loss!
When you say you keep carbs at 75%, do you mean they are 75% of your total calories? If so, that seems a bit high and can be the reason for the fluctuation. With a lot of carbs comes water weight and also, the more carbs you eat the less fat you burn. Carbs are the body's preferred energy source and if you lower the carbs your body will rely on the burning of bodyfat.
Of course, the type of carbs you eat also matters.
How much protein are you eating? If you lift weights, you should eat 1g of lean protein per lb of lean body mass to lose fat and not lbm.
Our bodies can do some weird things sometimes and water weight can definitely mess with you!