You know how in breastfeeding we say "watch the baby not the clock?" Well the same is (should be) true for weight gain. Look at the baby, not the scale. Is she happy? Alert? Does she have regular awake periods? Is she interacting with you? Is she starting to vocalize? Can she lift her head when she's on her tummy? Does she eat/poop/pee?
If you can say yes to all of that, then she's fine. Low weight or a slow gain might be an early sign of an issue, but unless there are other signs, there's no need to worry.
AND...you said that she was 9-7 at birth and 13-4 at 3 months...that means she's gained almost 4 pounds in 3 months...that fits in the 1-2 pounds a month "guideline".
As far as the most recent weight check, she was only "off" by 2 ounces...that could have been a wet vs dry diaper, maybe last time she hadn't pooped yet that day and today she had, maybe last time you fed her right before her weigh-in and today it had been an hour...many reasons why a weight could be off by a few ounces. She actually could have gained an extra 2 oz instead.
If her weight continues to track downwards, your dr might ask extra questions about her activity and development, and might start asking about your diet, your milk supply, etc. But as long as everything is still good, there's no reason to worry.
Now that I've said all that, I *completely* understand the worry! My second son is 3 years old and has only recently tripled birth weight (something most babies do by one year old!) Granted he is special needs with multiple medical issues, but weight gain has been a huge issue for us. My youngest son looks great, but he went from the 95+% at birth (9lbs13oz) to only the 50% at 4 months (15lbs). The dr was somewhat concerned at the drop in his percentile, so asked me to bring him in at 5 months for another weight check. He was still right at 50%, having gained exactly a pound. So for now we're okay. He does have reflux, so that's a reason to watch weight just a bit more carefully.
