So far my strength has increased but not muscle mass. You can best increase muscle size by lifting moderate weights with a high volume of reps and sets with very brief rests in between. If you see advertisements in magazines and health food store products for nutrition supplements that claim they can help you to bulk up, ignore them. They don't work.
Muscle mass has a genetic component.
You may not experience the same level of improvement as some of your friends, and the changes may be very slow. Don't worry about this. Lifting after you run means you are working out on depleted energy stores. You can't perform quality weightlifting this way. It would be better to run and lift on alternate days.