Why do we sometimes become allergic to something only later in life?

I have a shirt that is 100% Acrylic. I have worn it for years and then about 2 months ago I became allergic to it, but for some reason only my arms are affected by this. We also recently moved into a house that was newly painted. I have two other jackets too that have this affect on my arms.
I’ve also become allergic to perfume. I was able to wear it until about 8 years ago and then I started getting migraines from any perfumed products.
Why was it fine for me to use perfume and wear acrylic and now I’m allergic to it? What causes this and is it possible to ‘cure’ it?