So worried, she isn't growing

I will admit I'm a big time worry wart. Lately it had been in full force. My LO is almost 12 wks. She has had issues with reflux and will squirm and turn her head during feedings. She was on medication and similac total comfort. Because of her issues during feeding we switched her formula a couple of times. I went to her Dr. because I was worried that she was on the wrong path. He told me that her growth was slow but fine and to continue what we were doing. She is eating but less than she did. I've been tracking her weight at home and this morning she showed that she is down an oz from 3 days ago. I'm freaking out... I'm so worried that there is something wrong. That she will be failing to thrive.

Both my mother and my husband's mother have passed and I don't have friends with babies. No one to talk too.

Has anyone else had issues with there LO not growing well? What was your outcome.