the pill

Hi I have been on the pill for about a year + now. I take my pills every day at the same time and I am sexual active certain times in a year (long distance)with the same partner; and we use a condom every single time. Although a few times  he slides in for a few seconds and then puts a condom. My period usually comes on the 5th day of taking the placebo pill but it is 12 days late .I have taken 3 test which were all negative. One 4th day late and two on the 8th day late . More than a week before my period I had a cold and sore throats which required me to take antibiotics. Do u think the drugs might have reduced the effectiveness of the pill? Should I still take more test? Is there a possibility I might be pregnant? During the days I was supposed to have my period I put on a tampon  just in case it comes but when I took it out later it was just a tiny spot of blood and very ....extremely little darkish blood ( more like gouw ) . Can't consider this a period or spotting hence my fraustration.