Hi,
have you had exactly the same setup before? I mean:
- Same machine, not type of. Same piece of plane.
- Same route, same duration of sun exposure.
- Same amount of sun rays in the - say - 24-48 hours before.
If so, you can rule out that it was a factor from the outside.
This would leave the, improbable but not impossible, chance of a strange medical condition as said before: testicle, parkinson, diabetes etc...
Another possibility would be, that you changed something in your cosmetics, say: after shave, lotion, shampoo etc.
Last but not least: Some kinds of herbs, drugs and foods can change your sensitivity against the sun.
I wouldn't mind a lot if this was a single occurence, if this repeats itself, take a closer look and talk to your favourite medicine man :-)