Very common in first trimester. Weight gain at that time is typically due to bloating, water retention and constipation, so it's not unusual to have your tummy actually get smaller. Soon enough you will be gaining "baby weight" instead of bloated weight.
Yes in the 1st trimester I had the same thing, on some days I would be most bloated than on others. And my stomach would always be bigger at the end of the day. Plus I had very bad constipation problems then and that made my stomach even bigger at times! Don't worry that's very common.