pregnant or not...

I am so confused right now. It'll be a month tomorrow that I gave birth to my miscarried twins. I had the regular postpartum bleeding after I got out of the hospital for almost 2 weeks and then it slowed down to light bleeding. After that, my boyfriend and I had sex and my bleeding stopped completely.  Well on the 31st, we had rough sex since it's been awhile and my bleeding came back as soon as we were finished and it hasn't stopped  and now it's heavy like a period. I've been getting faint positive tests and I know it's not from old hormones because the lines are getting darker and they're popping up more quickly. I went to my follow up ob appointment a couple days ago and I told them it was possible I could be pregnant again. They gave me an ultrasound but they did NOT give me a blood test. They couldn't see anything in the ultrasound. All they could see was a little  fluid in me from my period about to start they said.(which that could be from my bleeding I'm having now) I don't understand why I'm getting positives that are getting darker and I don't understand why I'm bleeding heavy that's lasted 10 days. Could it be part of implantation bleeding AND an irratated cervix from sex? I also don't understand how they didn't see anything in the ultrasound when I'm getting faint positives that are getting darker.