Here is a link to possible causes of salivary gland infections. Interestingly, it states "Acute salivary gland infections rarely affect the minor salivary glands or sublingual glands. The acute conditions more typically involve the parotid and submandibular glands." Perhaps dentists are not as familiar with the swollen, crusted lip symptom since infections are more rare in the minor salivary glands.
The minor salivary glands lie just below the mucous membrane lining.
Since I developed this while I was on antihistamines for allergies, it is possible it caused an infection in my salivary glands (if I have an infection). Or viruses could be the cause as well.
"One of the most common settings for acute bacterial salivary gland infection to occur is in the dehydrated individual. This can result from simple dehydration but is most frequently seen as a result of medical therapy with diuretics and certain antidepressants and antihistamines that decrease saliva flow rates as a side effect."