A week of a flipped sleep-wake cycle after a long flight across time zones is a classic, correctable case of jet lag, not an illness. Your body clock just hasn't reset yet to the new day-night pattern here.
Next Steps
Get bright sunlight exposure in the morning and keep your room dark in the evening. This is the strongest signal for resetting your body clock. Avoid caffeine and screens for 2 to 3 hours before bedtime, and keep your wake-up time fixed even if you feel tired.
Health Tips
If your sleep pattern hasn't settled in another week, please see a general physician. This also applies if you develop low mood or anxiety alongside it.