Is This Even Possible? Long, But I Need Opinions

Hello, I posted about this a long time ago, and thought I had resolved this issue...Now its come raging back into my life. Bascially, I had a sexual encounter that could have led to pregnancy, and my next period was a full week or two early. It was normal flow/length, so I thought i'd wait for the next period to come on time or not. It did, and I have had 4 more on time and regular after that. I have always tracked my cycle, so the one weird period after that slip up really, really freaked me out. Although, over the past year I have lost about 40 pounds (i'm very underweight now for my height). Then, while trying to figure out what could have caused this, I learned that women can have periods and still be pregnant, and that this is pretty common? So at what would have been 12 weeks I took an equate hpt, which came out negative. I didn't use first morning urine, but the box said this only mattered for early testing. I would now be 22 weeks pg, and I have had no real symptoms. (no sickness, no breast changes, no missed periods, no weight gain, I still wear a size 0 or 2, no fatigue...In fact, the first month I thought I was pg I was so stressed I would stay up for like 24 hours at a time.) however, all things I have been reading online lead me to believe you can have periods throughout pregnancy, false negatives, no symptoms, and not show at all. Now, I would have gone to the doctor for a blood test to ease my mind, but I am completely broke right now, and I am still trying to find a clinic that gives them for free. I mean has anyone heard of this happening. I am just terrified that I didn't think I was pg all this time because of the periods, and didn't seek the proper care, which could have led to seriously harming a human life. I think this might be a guilt thing. Should I lay this to rest? Or buy another hpt? Or save up for a blood test? Please help.