I've Lost A Lot Of Weight Recently, But I Have

I've lost a lot of weight recently, but I have also been on a calorie deficit and every week the scale dropped. I got sick with a sinus infection two weeks ago and just finished my course of 500mg 3x a day for 10 days, 3 days ago. I became very bloated and uncomfortable half way through and now I am extremely constipated. My period is late because of the antibiotics but I'm still getting symptoms that it's coming. I'm extremely bloated and constipated and I weighed myself yesterday morning and I weigh 112 pounds now which means I've gained 5 pounds in a week... How is that possible while eating at a deficit?