Hi Tam!

Yes, it's possible to develop an allergy or sensitivity to a food at any point in your life, although true food allergies are fairly rare. My DH has always eaten nuts until last spring when he kept getting this rash on his wrist. He was also having a terrible time losing weight. Due to some other more serious reactions to foods, he went for allergy testing and you guessed it...he's allergic to all tree nuts as well as several other foods that he had been able to eat until then such as raw peaches, apples and celery (it was a bad reaction to peaches that caused him to agree to allergy testing).
I think you have your answer with the little experiment that you did. You felt better when you cut out the nuts and your symptoms returned when you added them back to your diet. You'll have to decide for yourself whether you like them enough to endure the discomfort that they are causing.