The papillae on my tongue swelled up and I got a canker sore after eating a plum. Other plums in the same box caused me no problems. What happened?

Trauma. It is possible that you bit your tongue in eating all of the dried plums and this causes your tongue swelling and canker sore.
Sensitivity. It sounds like a timing matter.
Could be trauma. Caused from accidental tongue biting especially since future plums from the same box have caused you no further problems. Hope this helps.