Just point to your head and say headache. Some people will offer different pain killers, and you can shit that down with "I just took my meds, now I just need to wait for it to kick in.".

I don't know if you feel comfortable with that, but I am pretty open when it comes to my diagnosis.

If I'm asked something like this, I'm usually straight forward about my coping mechanisms.
I have autistic traits and I wear earplugs because I'm very sensitive to noise or wear sunglasses because I can't handle the light right now, etc.

Most of the time it's a "oh okay, makes sense" kind of answer, sometimes I get asked more question about it.

~~I'm not autistic (that I know of)~~ (Edit: Just realized this is in the ADHD com not the autism com. No idea why I thought that. I do actually have ADHD) but I do have issues with light sensitivity so I'll often wear sunglasses inside bright buildings. If anyone asks I just tell them my eyes are extra sensitive to light. Nobody's really said anything about it after that. Most people honestly DGAF about what you're doing, they're just curious and sating that curiosity is enough. Those that do actually care and would make a stink over nothing are relatively rare and aren't worth your time or attention.