12 weeks and uterus feels empty?

Hi everyone, I had a missed miscarriage with my first pregnancy so now I am especially anxious about my current one. I had perfect 6 and 8 week ultrasounds with normal measurements and heart rate. The past couple of weeks I started feeling and looking more bloated, my lower abs stayed sore, and I felt some “fullness” in my uterus when I tried to suck in my belly that prevented it from going in all the way. Now those feelings have pretty much gone away and I sort of feel “empty” and like I can suck in like normal. I’m really small so there’s not much space for my uterus to be expanding without noticing. I’m afraid something is wrong. I have my 12 week appointment tomorrow but they won’t be doing another ultrasound until 13 weeks. Has anyone else had this feeling? Is it normal for your belly to seem to fluctuate and even seem empty sometimes?