Appendicitis will always happen yep, but it mainly happens when you're younger. Not at your age (they're pretty rare - I've read as low as 5% per NIH but can't find article).
Per NIH.gov
"Appendicitis is very common in adolescents, but here the proportion of cases complicated by peritonitis is small. However, because of the frequency of the condition, the death rate in the general population of this age is high, with a maximum at about the age of 15 years.
These findings are compatible with the suggestion that the appendix during the 'teens is particularly liable to obstruct and hence to become inflamed because of the large proportion of lymphoid tissue which it contains."
Per aafp.org
"Approximately 7 percent of the population will have appendicitis in their lifetime, with the peak incidence occurring between the ages of 10 and 30 years."
Per uspharmacist.com
"While no age is exempt, acute appendicitis commonly affects those aged 10 to 20 years."
I've read older adults rarely get them unless there's typically an underlying disease like a tumor or chronic parasite. Now the real elderly, that might be a different story, I'm not sure. I'm sure you getting an appendicitis right after your injection is purely coincidental though, Supe.