I'm not a doctor, but I wonder if you have a lot of stress in your job or personal life. Think about whether you have done all you can to reduce stress, stay hydrated, and get enough sleep every night. Also, remember that certain medications, such as antidepressants, may affect sexual arousal. Don't be afraid to talk to your doctor if you are truly concerned that you may be experiencing a health problem. It's possible that your body may just be experiencing a change. I'm 33, and my drive has definitely gone down in the past 3 or 4 years.
Best of luck to you.