new pregnancy?

Ladies, I found out at 16 weeks that I had lost my twin boys. But they measured to be 14 weeks. So they must have miscarried 2 weeks before I found out. I gave birth to them on the 11th and Im really not sure what my hcg levels were at that time. If I miscarried them  when I was 14 weeks, that was 5 weeks ago so my levels surely would be back close to 0 right? I have stopped bleeding and since then my boyfriend and I have had sex 3 times. I took a test today and it showed a really faint positive. Could that be a new pregnancy or just left over hcg?