What exactly causes "water weight" gain and could I have this problem?

Two years ago, I had a mild case of hypothyroidism. My original weight was 112 and it jumped to 119 in three months. Even after medication, I went to the doctor for the third time and there my weight was still at 119. Three days later, I went to my primary care physician to get a physical examination done and my weight was 114. Both times, my weight was checked by a digital scale. Although my hypothyroidism is treated now (it was not extremely severe), I have recently been diagnosed with vitamin D deficiency. My weight has been approximately 3-5 pounds heavier since last year (back to 119) and it is not going down. I even started working out a lot more and lift weights and feel like I have lost abdominal weight and weight in my thighs. However, I still continue to remain the same weight as before. I know that I occasionally suffer from dehydration and that I when I get sick (which I have been for the past few days) , I have trouble drinking water, leaving me to drink merely 8 ounces per day. Could this be a reason why my weight shows up to be more than I think it really is?