What are possible reasons for partial sensory loss on right side of upper lip and cheek?

Infra-orbital nerve: The nerve in question is the 'right infra-orbital nerve' [a branch maxillary division /trigeminal nerve (v2)=red area]. The nerve exits the skull just under the right 'cheek bone'. The nerve may have been damaged by trauma (old injury) or may have some compression on it from tumor/mass/infection in the maxillary sinus region. A sinus ct scan may be needed to find out the cause. Hope ur better!

Facial numbness: The more common reasons for numbness is trigeminal nerve issues, this can also be seen in small lacunae strokes and demyelinating diseases such as multiple sclerosis.