What Causes Breakthrough Bleeding After Missing Birth Control Pills?

I had breakthrough bleeding a little over a week after my normal period, due to missed BC. After that I missed my next normal period (probably due to the breakthrough bleeding) and am due to start for my next period today, but have no signs that my period is coming. I had a strange and very light pink discharge accompanied by light cramping that lasted about 4 days , this was eight days ago and that caused me to take a pregnancy test which was negative. My boobs have been incredibly tender for the last week and my nipples seem to be more erect than usual. For the last couple of days I have been having a dull ache in my lower back, and this morning when I woke up, my boobs weren t as sore as they had been in the past few days. I took another test this morning and it was negative. Is there an off chance that I could actually be pregnant? Or are my hormones just weird because I had messed up on the birth control pills? I had only taken the birth control for a month before I messed up and missed pills.