Not sure what to think, need encouragement

So I went to the doctors a week ago at 6w1d and they measure sac at 5w3d then I returned this week and they finally saw baby and heartbeat but sac is measuring on the smaller size.........doc didn't seem phased but Ultrasound tech said she was a little concern because the sac didn't grow much. Even though the doc wasn't concerned my gut instinct feels like something isn't right. My sac looks so small compared to all other pics I see of 6w1d......ughhhh if this doesn't work out it will be my 4th MC. I'm just overall feeling crappy.........the reason they scanned so early is because I am on progesterone and my history of previous MC otherwise they don't scan till 8-9 weeks. I return at 8w for another scan. No bleeding at this time. Just have a bad feeling.