Thanks for your query.
There are two possibilities might be there:
1)Melasma: this is condition where dark patches develop on face ( cheeks,forehead,chi,upper lips) etc.
2) Perioral pigmentataion : Frequently genetic, often increases due to factors like sun exposure, use of cosmetics etc.
You should visit a dermatologist
for the assessment of this.In either case , a broad spectrum sun screen should be applied regularly to prevent any further darkening.
help in lightening to some extent.
You can have oral supplements like VitC to help prevent this darkening.
Hope this is helpful.