Diphenhydramine (Benadryl) is an H1-antagonist, with unfortunate anti-cholinergic side effects: dry mouth (xerostomia), dry eyes (xerophthalmia), and also including urinary hesitancy and inability to void.
I would presume to think that the next best action would be to evaluate the bladder for bladder distension.
If overly distended, then a urinary (foley) catheterization may be warranted to void the patient. However, ordering a Foley first may be an unnecessary, uncomfortable, and likely painful procedure.
After examining the bladder externally (suprapubically), then fluids may be warranted.