Keep in mind that other areas of your life may also have an effect on your running. If your job or family life has been very time-consuming and stressful recently, you'll definitely feel more sluggish during your runs. Try to look for ways to reduce stress in your life and create more time for activities you enjoy, like running.
If you do cut back on your running and you're still feeling tired and sluggish, examine your diet and sleep habits -- they'll also have an impact on your running. Try to get eight hours of sleep a night and stick to a healthy diet.
Also, keep in mind that it's normal to have a bad run every once in a while. Don't let a bad run or two make you completely give up on running. Try some of these tips to get over a bad run.
Also see: When to See a Doctor for a Running Injury
Is It a Running Injury or an Ache?