You said it yourself. You're eating healthier than prepregnancy. That right there is going to account for a lot of it. If your doctor isn't worried you shouldn't be either.
Can't say it any better than this. Your doctor will let you know when/of it's a concern. Until then, don't fret. It's fairly common to not gain until later on, especially if you changed the way you eat.
I haven't gained anything and I'm almost 20w. No one is concerned.