Sometimes fatigue like that needs to be checked by a doctor. Last year I had fatigue so bad I couldn't sit up and found out I was dehydrated and my thyroid was low. Also make sure you are getting enough hydration all the time, not just when running, since as the temps climb it's easier to get dehydrated.
I have found that since the weather got hot here my endurance dwindled. I had gotten to week 9 and ran for 30 minutes straight but now can only do at the most 20 minutes before I have to break. SOme days not that much. Even when it's cooler, or I'm in the air conditioned gym my endurance is not as good as it was before hot weather. But heat has always had a negative effect on me to some degree. So I am just working with it and keep working at it.
Good luck to you and hope you can feel better about what you're doing soon!
Last is just the slowest winner."-C Hunter Boyd