Petite healthy child, dont worry. She's helping the growth chart stay "balanced" when the majority of kids are tipping the scales on the wrong end. My 6yo son is just over 30 lbs. We finally turned his car seat around at 15 months old even though he wasn't exactly 20 lbs yet. (He and I were much happier for it!)
When she's older, you'll be SO glad you're not having to haul out a kicking screaming 40 lb toddler! It's also very nice to be able to wear the same size for a whole year instead of moving up to a larger size every 4 months.
Only need to be concerned when her growth stops completely and regresses, ie, if she loses weight for no unexplained reason (ie, if she got the flu and lost weight one week, that's explainable, just be helping her gain it back when she's healthy again. UN-explained weight loss (or complete lack of growth) is what you need to be concerned about. There will be times that she wont gain any weight, but as you mentioned she is growing in inches, not pounds, and is hitting all the developmental milestones and staying active, alert, and engaged in her environment, so she's just fine!
Also, she may just be a petite baby, but once she's past the toddler (or early grades, or puberty, whatever) she might catch up to her peers. Were you/her dad (or anyone in the family) small or a late bloomer? Both my husband and I are so of course our son would not be a big baby either. My son will very likely be "underweight" into his 20's (Dad and I graduated high school barely over 100 lbs each - and the car seat we have holds up to 100 lbs, imagine that!)