My only concern with this is the anticholinergic properties with this drugs. You believe you are sero-negative Sjogren's and this is contraindicated for anyone with Sjogren's. This will compound the damage on any/all moisture producing glands...from salivary glands, eyes, joints, etc, etc.

I'm not sure why it's working for you, but my first thought would be the help with better sleep. Your body functions so much better as a whole with more fulfilling sleep. It would be interesting to know if the antihistamine properties have some sort of connection to nerve function. It's worth some Googling time to research.

How long have you been taking it, and how soon after did you notice the improvement?