What Causes Yellowish Tint In Semen Occasionally?

I notice a sticky substance sometimes in the toilet bowl during a bowel movement. Not always, but when I "strain". couple this with a low amount of ejaculate during sex (less than a tea-spoon full). Sometimes I notice what appears to be a yellow tint to my semen, and it is rarely but sometimes, thicker than I remember. ( I am a 48 year old relatively healthy male, married and monogamous, no other health problems.)