exercising itsself is a drug, otherwise you do something wrong.

Rules for a workout plan:

1. it includes various types of sport
2. rewarding regeneration of the muscles (hot showers, massage, rest)
3. doing the the sport at least 30 minutes regularly

--> this is the way to produce the "happiness hormones" which make you addictive.

If you are just tired and have followed the aboved rules you may just need more recovery or you just have a bad day for sport. in that case don't push you because it will demotivate you. That the last rule:

4. listen to your body.