This looks like a superficial irritant burn caused by waxing, which can happen if the wax is too hot or if the strip is pulled aggressively. The straight red line you noticed usually corresponds to the area where the strip was applied.
Next Steps
If the redness persists beyond a few days, develops blisters, crusting, or darkening, it is better to get it evaluated by a dermatologist so that proper treatment can be started early and marks can be avoided.
I regularly treat wax burns and post-wax pigmentation, so you can consult me online or visit the clinic for a quick assessment and personalized treatment.
Health Tips
Always ensure waxing is done with proper temperature wax and gentle technique, especially on sensitive areas like the upper lip.