Yes it is the birth control pills that are doing this to you. Birth control pills work by supressing ovulation but they also change your discharge.
Throughout each pill cycle, and continuously with Norplant implants and Depo-Provera, the mucous covering the cervix—the site where sperm enters the uterus—stays thick and sticky, making it very difficult for sperm to get through. This gooey impediment also acts on the sperm cell itself. It prevents fertilization by interfering with chemical changes inside the sperm that allow it to penetrate an egg's outer coating.
In normal, ovulating females, discharge changes many times during the month to either help or hinder sperm from entering the uterus. On the pill, these changes don't happen.
So, good news is, what you have is perfectly normal. Bad new, it won't go away until you're off the pill. Changing brands might help but it's doubtful.