Very very faint positive that disappeared

Hello, I need some advice, basically I had my period (5 days early) 2 weeks ago that lasted for 10 days and was quite heavy and I had a few big clots. I then had nothing the Monday just gone and thought that was the end of it but then started spotting on the Tuesday and full on bleeding again since Wednesday. I've been to see the doctor who did a pregnancy test that came up with a very very faint positive that then disappeared. Even the GP wasn't sure of the result but referred me for ultrasound. Has anyone had a very very faint line cone up that then disappears? X