Dryness in mouth! Why is it happening?

I had pigmentation in my buccal mucosa since 1-2 months, showed various oral pathologist/dentists they're saying its normal. I am okay with that but the symptoms i am facing majorly is, the buccal mucosa in both the sides where upper set of teeth is touching, feels so dry and uncomfortable, when i open or close the mouth, or say when i drink water, post that it feels abnormally dry. Please help me why is it happening? Attached pic of left and right mucosa.. I am non smoker, non tobacco user and no alcohol since long time too. 2 weeks ago i had completed ab phylline 100mg along with prulastin tablet because of my pre asthma symptoms. I guess a week after that these issues came up. Its getting so uncomfortable for me to deal with this, after drinking water it feels so rough. Please help me out. Last time 4-5 weeks ago when i checked me pre sugar it was normal only 90. In December had done a hba1c test that too was normal. I don't eat much sugary things now as well. Please help me!