Do Delay In Periods And Swollen Breasts Indicate Pregnancy?

Hi there,
I am a 19 year old female,will be turning 20 in June.
I was on the injection for 2 months and my next appointment was the 3 January 2014.
I recieved my period and ended in january the 5th 2014.
I am just worried because i have been using the pull out method when we did not have a condom a couple of times and i did not get my period yet and it is the 16th already.
I took a pregnancy test on the 4th of feb this month and it came out negative.
My breast are swollen and i been getting white thick milky discharge ,at times its slimy too.
My bottom has gotten bigger and my jeans and shorts are too small around that area.
I also have terrible constipation and fine pimples on forhead with bigger pimples on my cheeks.
I have always been naturally skinny and have weighed 47 for years.
I have no idea whats going on and need your help?