Antidepressants and certain medication can cause premature ejactulation. Are you taking either felius? Also, one thing to point out, is that premature ejaculation is the most common sexual problem in men, affecting 25%-40% of men. Most men experience premature ejaculation at least once in their lives. Often adolescents and young men experience "premature" ejaculation during their first sexual encounters.
One key point, don't dwell on it.
I advise you to read this other thread, it may not be along the lines of your issue, but has similarities and very well acknowledged information a lot of men should know.
http://ehealthforum.com/health/topic50567.
html
Hope this helps. Good luck!