A single pound of fat is roughly 3500kcal. So unless you ate 10,500kcal OVER your baseline intake, there is absolutely no way that is true weight gain. It could be water weight, hormonal changes, bowel movement issues, etc. There are dozens of factors that impact weight and might be reflected on the scale. Additionally, the more you lose and closer you get to a healthy weight, the more likely you are to see those fluctuations as the week to week loss slows down. Keep sticking to your plan and doing exactly what you're doing and you absolutely will see the scale move again. And congrats, 90lbs is amazing!