Multiple days of implantation?

I was just wondering if anyone has experienced the same type of thing as I have. . . I am now spotting the third time, I do it for about 15 hours and a week between each time. After the second time I got an ultrasound and they couldn't see a baby, we thought I was 6 weeks but I was measuring about five due to what they think was that I ovulated late. They showed on the ultrasound that I had more blood in there trying to make its way out, which is what this probably is. They've told me its implantation bleeding. It's just so strange to me that I've been experiencing implantation bleeding for like three weeks now. Anybody else have experienced like this?