Is it possible that my milk is not nutritious enough?

Yes: Yes, but it is very unlikely. Breast milk usually has 20 kcal/ounce, which is why most infant formulas are made to have the same caloric ratio. If your child is content and gaining weight, chances are that your milk is fine. When breast fed infants don't gain weight, the problem is more often lack of milk or feeding techniques and not the caloric quality of the milk.

You are what you eat: Your milk is your product. The more nutritious you eat, the more nutritious it is.