Bed rest - weight gain stalled

At 23 weeks exactly I weighed in at 148 lbs. two days later I was ordered to be on bed rest, and literally the next day my weight went down to 145 lbs. Its pretty much stuck at this number for the last 10 days. I eat normal meals, I drink a lot, I obviously have zero physical activity... So why did I lose weight and stopped gaining? Up until the bed rest I was steadily gaining.

I understand that bed rest will eventually cause muscle loss, but the 3 lbs drop happened after just 1 day of being down.

Any insight on that will be appreciated!