Am I pregnant?

I had sex with my boyfriend 13 days ago, and my period was supposed to start about 4 days before sex until 3 days after. But it hasn't come at all, and 3 days ago, I started feeling a pulling sensation in my lower abdomen, but it doesn't feel like it usually does. The pulling sensation only occurred when I exerted myself, like lifting heavy objects. Yesterday morning, I had brown threads and a pink, transparent discharge. Then, in the evening, nothing at all. This morning, it was very bright red but a little bloody, and now it's a transparent pink-light brown again. Since yesterday, I've also had nausea, lower back pain, and my bladder is pressing every 40 minutes, but hardly anything is coming out… could this be implantation? The pregnancy test today was still negative.