Toddler suddenly becoming allergic to certain foods

[Poppedcorn]

My son is 2yrs 8mths. Just in way of background, when he was 6mths old and started weaning, we discovered that he had a dairy allergy, and at 18mths we were able to start reintroducing dairy products. He can now eat food containing milk, but if he drinks milk itself or eats yoghurt, he'll get a bad stomach the next day. So his allergy has improved, but not gone. He wasn't found to have any other allergies.
However, in the last few weeks he has suddenly erupted in hives on his face and neck after eating mango, nectarine and now strawberries. He's been eating all of these since he was a baby, so it's strange that he's suddenly become allergic. Or is this quite common in someone who already has an existing allergy?
I'm gutted for him as strawberries are his favourite food.
If anyone has any experience of this, I'd be really interested to hear.

Its not strange to suddenly become allergic, ds was 4 when his allergies really started. One weekend when he was 4 he had a reaction to sesame on the friday where he was violently sick and then on sunday he had pecan pie and his face swelled up and couldnt stop sneezing. After that he couldn't eat any tree nuts and peanuts, although eventually he grew out of the peanut, but not the seeds or treenuts.

I have a similar experience to bruffin. Houmous was one of DS's favourite foods, he ate it many many times before reacting to it. He tested positive to both sesame and chickpea allergy.