I would recommend medical attention, however it might comfort you to know that amphetamines can cause dysfunction of certain genitourinary mechanisms - I only know this because I have more than a little personal experience with it. I am not saying don't worry, just because I never got it checked out, for I really should have.
Anyhow, here are a few things I would recommend to help, and some others might think I am a little whacky, but I know enough to give some good advice:
1) Keep yourself hydrated with good old-fashioned water, consistently through the day and night.
2) Don't let yourself starve; keep well enough nourished as is possible.
3) Look into non-stimulant laxatives like docusate; this will help ease your bowel movements with less discomfort than with other types of laxatives, however prescription lactulose would be ideal. You want regular and easier bowel movements, as the bowel and rectum can retain more feces and harder feces as a result of amphetamine use - this retention causes pressure on the prostate, which is extremely erogenous and I hypothesize may cause stimulation of the physical sex organs and their processes, potentially triggering the unusual arousal responses and physiological sex processes. I do not believe anyone took the time to study an amphetamine user's genitourinary function, so all this is informed guessing based on my personal knowledge and education as well as my personal experiences which mimicked yours, where for me I would often generate pre-ejaculate easily and involuntarily and I would often find that urinating during or right after a bowel movement would produce semen - this felt like urinating, not very remarkable, but occasionally had a bit of a burning feeling associated.
4) Drink cranberry juice regularly for a couple weeks, and when symptoms relieve themselves, follow up with maybe a couple cranberry juices here and there throughout the week.
5) Use Preparation H wipes with witch hazel, just to keep the anal area less likely to develop hemorrhoids, fissures, and discomfort.
And, keep in mind that amphetamines are sympathomimetics, which means, more or less, they influence other areas of the nervous not just the CNS; and, the kicker is that sympathomimetics cause mucous membranes to secrete less fluid, leaving them dry and less healthfully functional than they would be - this involves all mucous membranes of the body, including the gastrointestinal and the genitourinary systems. Amphetamine also works as an efficient diuretic, increasing the demand for more water and more regularly for optimum function under the circumstances. Water is the true key - hydrate and flush.
