Does dieting affect my breastmilk?

Not much. As human women our bodies will leach nutrients from ourselves to preserve the nutrients in our milk. What this means is that your breast milk will have adequate proportions. What will happen, however is that your body will become deficient. This is particularly important for calcium. Try a supplement or even continue your pre natals.
It might. Eating healthy is certainly best for you and your baby, but if you want to lose weight, please see a nutritionist who can properly guide you in this process.