I'm not a doctor, but my father is, and I read a lot about medicine for reasons I'm not sure of-
First off, some anti-depressants make you sleepy, so it may make you even more sleepy. Of course, ask your doctor etc. Some medication lowers testosterone as well.
Hopefully you don't have a pituitary tumor (Doubt you do though) since that causes low testosterone at any age. Same as mumps, which may (low chance) affect the testicals.
Naturally, depression also may cause constant fatigue.
What is weird, is that you haven't lost libido, which usually goes along with "it".
Like others have said, good nutrition and good sleeping schedule is most important.
I'll read more about it, Hope everything goes well.
Note: I'm NOT a licensed physician.
Here are some good articles:
http://www.livestrong.com/article/238742-what-are-the-causes-of-low-testosterone-in-younger-men/
http://blog.americanlongevitycenter.com/low-testosterone-in-young-men/
http://www.tandurust.com/endocrine-system/low-testosterone-young-men.html
http://www.livestrong.com/article/440332-why-do-i-always-feel-tired-and-weak/