If you always feel tired post-nap, it likely means that your naps are too long.
Or perhaps you’re not allowing yourself enough time between waking up from a nap and going back to bed at night.
Taking shorter naps during the day should help alleviate this issue. 
If not, it's possible that underlying conditions such as insomnia or chronic fatigue syndrome might be causing fatigue upon waking from a nap despite its short duration.
Try talking to a sleep expert to assess your sleep patterns and find out how you can improve your night’s rest.
Furthermore, certain medications (such as antidepressants or designated sleep medicine) may also contribute towards tiredness after taking a nap. 
If you’re concerned about feeling tired after a nap, it may be time to seek some medical advice.
Ask your GP about potential side effects related to any medications you’re taking, particularly those that have been recently prescribed.