Why the sudden allergies?

Yesterday, I had a sudden allergy attack. Started sneezing and couldn’t stop, eyes watering, everything itching, nose swelling, the works. I don’t know why. The only times I have had this before were when I had a sudden puff of dust in my face, like when I’m mixing henna powder and am not careful. I took a Claritin and it got much better but wasn’t fixed. That medicine has always completely fixed the problem of dust reaction, but not today.
I work from home. I haven’t left the house since three nights ago. I sit in the same place most of every day. I hadn’t even had any food or drink just before the attack. And now I’m still cloggy and itchy and it’s not time to take another dose yet. Why am I all the sudden allergic to the exact same surroundings as I’ve had for months?