Is it possible that running/aerobic exercise can actually cause depression?

There is a lot of research purporting that running or intense aerobic exercise is the best anti-depressant, which I noticed first-hand when I started running 30 miles/week over two years ago. But recently I’ve noticed that following a 5–6 mile run, my mood and libido drop substantially; I’ve experimented with this and have found no other cause. I’ve read several articles indicating that running more than short distances can lower testosterone, which in males has a negative effect on mood. However, if I don’t run, I feel like a fat lard, gain weight, and lose energy, but at the same time, my mood and libido return to normal levels. So I’m sort of at a crossroads—I don’t want to stop running, but I also don’t want to get depressed. Have any other males had similar issues, because I feel what I’m experiencing runs counter to the prevailing research on running and depression. Should I look into hormone replacement therapy?