Bleeding!?

I found out I was pregnant the morning of the 12th, I updated my glow app to pregnant and they said I should be about 5 weeks along. I woke up this morning to brown spotting, I knew this could happen and didn't panic. I wasn't until several hours later that I started bleeding red and it was more like a first day of a period. So I went to the ER and they did a blood test that came back with an HCG reading of 101.4 putting my pregnancy at about 2-3 weeks instead of 5. Should I be concerned about the difference between those estimations ? The Doctor told us that bleeding is common sometimes this early on and is likely because we had sex.