diagnosed with a nut allergy at the age of 53?!?

my mum has never been allergic to anything. but few weeks ago... she was sat down watching tv. opened a bag of hazelnuts and had a few... not long after she calls me as she was not feeling very well... i go over there and her face was swelling up and airways... so she took piriton and then went to hospital.. she had blood taken and results came back she is highly allergic to all nuts.. including coconut.. (her favorite!) she has never ever had any problem before. how has this happened to her? what is the cause for this sudden allergy?

thanks in advance.