Clearly a case of CRPED. Since you can't find anything in literature, here is a quick description of the pathomechanism:
It's a misdirected immune reaction to the diphenhydramine. Antibodies are produced and instead of attacking the aggressor they are turning against the erectile elements in the male reproductive organs. It is therefore classified as an autoimmune disease.
In the worst case scenario the antibodies will eventually attack your penile arterioles, causing penile hypoxia and eventually leading to necrosis of the penis.
Picture of Penile Necrosis (Warning: Not for the faint of heart!)
You need to have an MRI of the Penis and Scrotum asap which will reveal the damaged tissue.
Try to feel if your penis is still warm. During the initial stages of the immune reaction there should be increased bloodflow, which will become apparent by a warmth emitted from the penis.
Once this stage has passed, the bloodflow will be considerably decreased, leading to the aforementioned hypoxia. During this stage your penis will feel considerably colder than other areas of the body. Should this be the case, you should immediately seek medical attention in order to avoid complete necrosis of the genitalia.
Alternatively you can measure the penile temperature by inserting a thermometer into the meatus, eventhough this is considered a very unpleasant diagnostic procedure.
There are several things you can do to prevent further damage:
-If the penis is still warm, try to reduce the infection by stroking the genitalia with ice cubes (CAVE: Excessive cold can cause frostbite, so either stroke intermittently OR if you're planning to leave the ice cubes there for a while, make sure you keep a towel between the ice and the skin.) Ibuprofen might also help to reduce infection (~400mg, NOT several blisters as it was the case with the DPH!)
-If the penis is already cold: Take a hot bath for several hours and manually stimulate the penis with massage techniques. This will allow for some blood to flow back into those arterioles that have survived the acute immunoreaction.
-Most important though, since what you will be trying to do is to stop the immunoreaction, is the following procedure. (I'll try to put it into easily understandable terms

We can achieve a reinstatement of the reproductive functionality by "reminding" your body of the attacked tissues function. Meaning you need to reach an orgasm by any means possible.
A proven method to achieve this with persisting lack of erectyle function is mechanical prostate stimulation. If you cannot internally reach your prostate with your hand, you can either have your girlfriend (slash wife slash best buddy) assist you OR you can use an instrument as aid. Here's an excerpt from wikipedia:
Aside from the advice I've already given you, I can only urge you to pray to Lord Jesus and our dear father in heaven. All modern medicine will not cure a patient if he is without god's aid. So hold your spirits up (no pun intended) and pray pray pray for god to grant you a second chance after your grave mistakes, the mortal sin you have committed.
Thou shalt not pleasure thee with thy anticholinergic medicinal preparation!!!
^^