If you are taking fine care of your rabbit, including but not limited to proper diet, clean environment, and general good husbandry practices, it would be highly unusual for pneumonia to suddenly appear out of nowhere. A mild URI is much more common than full-blown pneumonia. You’ll see here (https://wabbitwiki.com/wiki/Pneumonia) that the symptoms of pneumonia, if developing, actually are associated with a URI, and what anecdotally I’ve seen is pneumonia crops up with the rabbit is not properly treated for a URI, paired with poor rabbit care practices, allowing the infection to get so bad that we see pneumonia develop.
Also, re: forum reading and getting anxiety (which, absolutely been there done that on my end… it is scary to read about other’s experiences!), remember that people post and ask questions online usually when things are going wrong, not well, unexpectedly, and/or they are simply lost and overwhelmed. As a consequence, we can get a skewed perception of how prevalent some of these conditions actually are and how severe their presentation/prognosis may be. Online, you’ll find a lot of the “worst case scenarios” and rarities, since those experiences are what draw people to seek help online.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.