Hi
If you experience redness, swelling, itchiness or any other irritation or discomfort on your skin around or beneath your watch or band, then Apple recommends that you remove your Apple Watch and consult a doctor / physician before wearing it again.
As you are evidently aware, skin reactions can be a result of various causes including an allergy to nickel.
More information, including tips from Apple:
- Cleaning and wearing Apple Watch - Apple Support
Includes "Tips to avoid skin irritation, or if you have allergies or skin sensitivities".
- Wearing your Apple Watch - Apple Support
Details the component materials of Apple Watch cases and bands.
- See under "Skin Sensitivities": https://help.apple.com/watch/#/apdcf2ff54e9