Hopefully you've been adding strength training into your cardio and stretching routines. While dumbbells are great tools to use to target your muscle groups, they're not the only tools. Try to explore other options such as weight-lifting machines at the gym, medicine balls or kettlebells, inexpensive resistance bands, different sized exercise balls, and you can also do exercises that require no equipment at all.
Switching up 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. Strength training is definitely important, and varying your equipment is a great way to make your time worthwhile.