since then I have a frequent need to urinate, which is sometimes painful and difficult. What's going on? I got a prostatitis?

Well, alcohol do irritate the prostate gland and can cause prostate congestion. 
I think you might be attacked by prostatitis. 
I suggest you to visit local doctor to have your condition diagnosed. 
To cure prostatitis, you may try Diuretic and Anti-inflammatory Pill.
It is a kind of traditional Chinese medicine made from almost 50 kinds of elements and each of them have their unique function in curing genital system and urinary system disesases. Some of them can promote blood circulation and dissolve stasis, some can promote Qi and stop pain, some can kill bacteria, virus, mycoplasma and chlamydia, and some can clear away heat and dampness. They all aiming at eliminating your symptoms to cure it.                                    
Released in 2011-09-17 10:33:28