Might not be any of these things, but given you are thinking about environmental changes:

Changed cleaning chemicals (home or school)? Washing powder? Shampoo? body lotion? You wearing new perfume? Candles burning at home? New clothes that haven't been washed in your own powder? A pre-made food that was safe has changed it's ingredients and isn't safe anymore? Weather?

I don't have a fantastic sense of smell (my DH does) but I still get headaches, behaviour changes, etc from chemicals/smells - only I don't know why, where as my husband gets less sick but whinges bitterly about the smell and why isn't it bothering me, and then I go "Oh it IS bothering me, that's why I have a raging headache all of a sudden!". Last week when I picked my DD up from daycare I could smell their washing powder intensely everywhere inside AND outside in the yard. My eyes were burning and I had a headache by the time I left (5 mins exposure). It's on my list of possible triggers for issues DD was then having the next day... The thing that boggles my mind is that in my house I have the crappiest sense of smell, but when I asked the cook if they were doing washing she gave me a very puzzled "How the heck do you know that???" look as she said yes, apparently the incredible stench of washing powder was not bothering anyone else.

Best friend's DD (with food and chemical sensitivities) had a complete breast refusal for days as an infant before my friend realised she'd started using a new body lotion. And same child had a night of prolonged screaming until mum took her clothes off and realised as she did it that the clothes were hand-me-downs washed in someone else's powder....