Late period, Can I be pregnant?

Hi ladies so im three weeks late and still no period. I already have a one year old and want to get pregnant again. I'm pretty regular with my period I usually get it every 8th each month. The point is I have taken various pregnancy test even first response and they all come back negative. I've been with really bad sore nipples these past few days and even lots of discharge but still no period. Can I still be pregnant? Even though test are coming back negative? If so how many weeks late did it take for you to have positive test?