I just went through something similar to this. I even got a positive HPT. But woke up and when I went to the bathroom I would have blood clots come out (mucousy blood). I only am needing to wear a pantiliner but my dr says it is a chemical pregnancy. That is when you lose the baby in the first 6 weeks of pregnancy and if you werent trying to get pregnant you would probably not even know this happened. The odds of you still being pregnant are pretty low. Good luck to you =)
Some women do get what they think is a "normal" period when they really are pregnant. But also as the person above said it could mean a chemical pregnancy. I would maybe just take a test to see what it says to be on the safe side. The symptoms could also mean you have a UTI. I know most people associate burning with urination with a UTI but you can also have frequent urination, abdominal pain, and blood. If you take the test and it comes out negative then I think it's time for you to call the doctor. He may just want to do some bloodwork to make sure you are not pregnant and may want to see if you do have an infection. Hope everything goes ok for you.
I had similar symptoms during very early pregnancy which I unfortunately lost at four and a half months last month passed. Although I did not have the spotting until later into the pregnancy at the beggining I had the pressure feeling on the right side which is what made me get tested and I worried through the whole pregnancy up until I lost the baby. It is highly possible You're pregnant and I would have it checked as soon as You can. It is possible that you could have twins and lose one earlier yes although it is not a high percentage. Goodluck