I have a preemie who is a slow grower from a weight perspective. But she's long and skinny. Basically, our Ped has been watching this from visit to visit and factoring in everything from a growth perspective-weight, height, head circumference. My daughter is an eating machine and is super-active too. The rule my Ped and I have come up with is that as long as my daughter is consistently growing in all areas, we aren't sweating the weight issue.
I wouldn't be concerned with the blood work either unless Dr. tells you there is something to worry about. I think it's good practice to check for known issues that might be responsible and if you don't come up with anything, you get some piece of mind. The other thing to remember is that every kid is different and they will grow like crazy one minute and plateau the next.
Personally, I tend to look at the averages among all our visits since we see big gains at some visits and little ones at others. The only thing that would alarm me is if she lost weight, say, a pound and hadn't been sick recently.
You are doing great, J., don't panic! Just keep doing what you are doing and let the Dr. tell you if there's anything in the bloodwork that is concerning. If not, then see what happens at the next visit. You will probably have a big gain at the 15 mo visit if she only gained an ounce for this one.
;)