errectile dysfunction or other medical problem?

hello doctor. i m a 21 year old male. i have been sexually active 2 years ago, never had any problems before. The last time i had sex with my ex gf, about 4 months ago, we use to do it usually 2-3 times a week, and i never had a problem before. about a month ago, i met this very attrictive girl from school and the first time we tried to do it, i had an errection all the time, just when it came to it, i had to put a condom on, and i lost my errection, we tried again later after 3 weeks, and i had premature ejaculation after about 2 minutes, and i did it 3 times with her tht day. but the sex was not normal. it was very short lasting but i did not use a condom this time. i have never had any serious medical problems before, however, i experience frequent urination in the past 1 month plus, sometimes i feel i m errected and in foreplay and i need to urinate, and sometimes the urine is not much, and i feel i need to force the remainin urine out. i have also been feeling very weak, lazy, and energy-less. lately, i have been sleeping for 12-13 hours a day. and i still never felt rested.
the only infection i ever had regarding my genitals, is once i had candida penile infection, after taking a 2 week course of amoxylin antibiotics due to high fever. so the doctor presribed me NIZORAL and after sometime it went away. i tried using Nizoral the other day, and i felt i had better stronger errections. however, when i stop using nizoral, i feel its weaker.
I also feel that my penis is very dried, and during forepaly , i usually got more lubricated naturally, and now, i feel , hardly any natural lubricant comes out. and i feel my urethra is constricted . i have bloated stomach, and feel like pressurised at the abdominal region. i have dark unhealthy stool, and i have diahreaa when i drink too much beer. i also have chronic bleeding from my pharynx/nose region, usually in the morning when i clear my throat.
i decided to see a doctor, and do a urine test for my frequent urination, and the urinalysis did NOT  contain anything abnormal, no wbc, no blood, no glucose, normal ph 7.2.
what could be my problem? sorry for writting so much and taking your time, but i really need ur help.
i m worried and am feeling very depressed because of this problem i m having. has also killed my sex drive lately. i m a medical student, and m undergoing exams at the moment, so being stressed is normal i guess. but i have been stressed in other situations before too, but this problems never happened before. i m very concerned about my errection problem as i use to be active sexually before, and my gf is worried about my problem.
thank you

[Caryopteris]

I think you have a systemic yeast overgrowth from the antibiotic.  It is causing lethargy.  I would suggest taking a probiotic such as Align.  Drastically cut back your simple sugars/white flour type foods.  Eat lean protein and leafy green vegetables to improve your health.  No beer until you have gotten your yeast overgrowth back to normal levels.