Please help!!! Possible miscarriage or ectopic?!?

I believe I am/was pregnant due to my symptoms, but for some reason have not got a bfp. I have been having serious abdominal cramps and backaches for over 2 weeks now and they have gotten worse the last few days. I have also had nausea, sensitive teeth, headaches, much larger, sore breasts, and my period is 3 days late.

But tonight I just found a huge blood clot that was super dark brown and very thick. But I only have had like a tiny bit of spotting on my pad since then. It was not like my periods at all, so now I am very worried that maybe I had a miscarriage or that it is an ectopic pregnancy which would explain why I still wouldn't be getting a bfp. I also read that sometime dark blood can just be old blood and my husband and I BDed last night, so could it be that I am pregnant and it is just old blood that came out?

I am just so confused and worried. I know my body, and I know something is going on. I have been having too many symptoms for way too long for it to be nothing. My back is aching really badly and I am still cramping. What should I do???