Mama weight gain...

Ok...so I started this pregnancy at 133. Mostly muscle as I'm a Marine and have to be pretty fit. I've been running some still, eating normally (never been a big eater), and I swear I feel huge. Doc says baby is great, measured perfectly the last ultrasound, and that I shouldn't worry that I've only gained 8 lbs. 8 LBS AT 26 WKS. My 1st baby was 5 lbs 1 oz at full term birth and he's always been my little guy, but I just worry. Doc said eat an extra yogurt everyday. Why are my babies only 5-6 lbs? Why can't I gain weight? Did I lose my muscle and that's why I'm only at an 8 lb gain? Anyone else worry over this stuff and try to think of every reason possible?