Anybody know anything about photosensitivity?

There are many possible causes. Many connective tissue conditions have photosensitivity. There are genetic conditions that may show up later with photosensitivity. See the dermatologist who will ask many questions and may do some lab work even if you don't have a rash at the time. They will likely want to see you back if and when the rash comes back. Got to wear that hat though, some of those conditions have body wide systemic ociations and then the sun exposure can actually activate the condition and cause internal inflammation.