I'm seriously stressed and I know it's not helping

So I am 12 days late, and I am on the pill. I take it perfectly because I am constantly worried about getting pregnant. My boyfriend and I don't always use condoms but he always pulls out in time. I took a test on Monday-10 days out- and it was negative, but for the past 2 days my breasts have been tender to touch. I don't know if it would have to do with PMS? Or what is going on. That is the only odd thing i have been feeling lately. I have started school and work a lot so I know I'm stressed and I've also been stressed about missing my period obviously. Does anyone out there have any advice for me on what could be going on??