My left eye has been mildly tender for a day or two, but this morning I woke up with a red line under my eye that appears like reddish purple eyeliner. Underneath the eye, it is also puffier than under my other eye. Please help me understand what this might be attributed to. I have not been able to find any answers online and have only seen this "red eyeliner" symptom addressed in the previous question on this page.
With sudden onset the most common cause would be infectious: stye, infected cyst, skin infection. etc. Use warm compresses and call an Eye MD if not better tomorrow.