I do think you need to relax then, the 6th percentile is great (we hang out between <0 and 1.5!, we were only off the charts at one appointment). She is meeting her milestones, and her doctor is not worried.
Butter works great, I butter everything for DS, especially when he was under 1 year. It also tastes good on veggies and in stuff like potatoes and pasta dishes.
Pregnancy can make existing worries seem HUGE, I have anxiety to start out with and since I can't take any good meds while pregnant, it seems to run away with me some times. I'm just a super worrier.
Let me tell you, my 18 month old son is wearing 3-6 month pants! (we potty trained, undies are slim!) He is one skinny dude!