Feeling fatigued could mean a number of things.
1) You might not be drinking enough water before and during your workout.
2) You were tired/drained from work (if you work out in the evenings)
3) You didn't get enough sleep (if you work out in the mornings)
4) Your diet is inadequate
5) The workout is kicking your butt
6) You're bored with your workout - happens to me
Pushups are pretty intense so I can see those kicking your butt, but overall I wouldn't automatically assume it's mainly due to 5).
I posted some ideas that don't require weights in your journal. I can find some more for you.
Everything should be made as simple as possible, but not one bit simpler - Albert Einstein