What Causes Brown Colour Semen?

Hello I went in to see my regular doctor on Monday because I have brownish semin. He gave me a colon finger check and asked me a series of questions, and sent me home with some antibiotics and said it's common and to check back with him in 3-4 weeks, and in the mean time have sex with my wife or masturbate more than normal to clear things out. . It's been 4 days since then and it has gotten much darker. At first it was mostly normal semin with some dark, now it's mostly dark. There's no pain when ejaculating, but my testicals have been more sensitive lately and have felt a lot like blue balls. Also it doesn't hurt when I urinate and no blood in the urine, he did check for that too. I'm kinda freaking out here. I'm 35 yr, 6'0" 188 lbs no real medical history to speak of other that some broken bones. Also I haven't had sex with anyone other than my wife in 11 years. Your advice is appreciated