Hard to answer that question from just the info you've posted.
But I can say that normally sleep is controlled by our natural circadian rhythm. A number of mental and physical systems are designed to sleep, rest, and rejuvenate at night when it's dark. And be active during the day when it's light.
Light is normally the key to this circadian cycle, but it doesn't seem to be that way with you.
It's possible to switch this day/night cycle of course, and in fact many people who work the night shift do exactly that. But that doesn't change the fact that our mind-body systems are intended and work best for sleep at night.
What you are describing could be some sort of a circadian rhythm disorder. This would be something that a healthcare professional could diagnose.
A second system that controls sleep is the homeostatic sleep drive, which reinforces the circadian rhythm.
If and when you have these two systems synchronized and working together, they reinforce one another, and sleep becomes easy and natural, in fact practically irresistible.
It may be possible for you to entrain your circadian rhythm back to a night-sleep/day-wake pattern by keeping a consistent schedule. And especially have a set wake-up time in the early morning.
After about 16 hours of nonstop wakefulness, your homeostatic sleep drive should powerfully reinforce your circadian rhythm to sleep.
Taking these sorts of actions with a disciplined sleep schedule is something anyone can do. It might help you if you haven't already tried it.
Still, you may want to mention this to your doc in case you have some underlying medical issues that need to be addressed.