I am severly allergic to almonds and don't understand how?

I used to eat almonds like they were candy but the other day I only ate 3 and my throat closed up. Turns out I have a severe allergy to them now but i don't understand why as I used to eat them so much and it is only too almonds I don't have problems with peanuts or pistachios as I have eaten them since the almond situation. I was wondering if anyone else has had this happen and if anyone knew why it might have happened. I have always heard nut allergy as child and then growing out of it but not getting it as an adult.