Hopefully you've been incorporating strength training into your workouts along with cardio and stretching. Lifting dumbbells is a great way to strengthen specific muscles groups, but it's not the only way. Try to explore other options such as weight-lifting machines, resistance bands, medicine balls, gym balls, and exercises that require no equipment.
Varying the equipment you use will inevitably work other muscle groups, and because they make your muscles work in different ways, you'll be strengthening them more thoroughly, which could prevent injury. It's also smart not to stick to repetitive motions, since that could also result in a pulled muscle.
So the next time you're at the gym, if you're not sure how to use the other types of equipment, ask a trainer to show you how. Gyms also offer strength-training classes, where you'll learn tons of new moves that don't use dumbbells. You can also pick up a resistance band and do exercises at home. Strength training is definitely important, and varying your equipment is a great way to make your time worthwhile.