This is very common in fact.
When you lie down for a while this allows gravity to help the acid in small amounts up the throat.
Small amounts can even be aspirated into the trachea.
This is well known among internal medicine doctors...we treat it all the time.
If it were allergy or infection for example,,,,it would not be worse at night, or aggravated from lying down necessarily.
The other part of your question is interesting...if it were allergy..the benedryl should help, not make it worse.
The fact that benedryl does not help goes for reflux.Asthma
or allergy ought to at least get some benefit from benedry.
Of course, you can never be sure there is not some lesion, tumor etc, in the throat..and if you had chronic sinus blockage this would certainly account for it, but you are not describing this.
If you test it for two weeks and you know...fine..if it does not work..then you have to look for these other explanations.
Good luck again
Let me know if you have further questions, details or need clarification, just use reply.......