Small Town gives me a headache

OK- so I've been traveling a lot lately and getting into headache-inspiring situations, but I haven't had ANY headaches (of any kind) at all until I come home to Small Town.

• Chicago - headache on the drive there (from Small Town, of course)
• Mexico City - bad air quality, hot weather - no headache
• San Diego - sunny, windy, bad allergies, a cold - no headache
• back home to Small Town - headaches return!

I am more convinced that they are allergy-related, but seemingly only for certain allergies (?) No migraines yet, but I feel them coming on daily...