Very sorry, I wish you had gotten better news from your cycle. For me the clinic considered it a negative and I was told to stop meds.
I wondered if it meant some sort of implantation had occurred and failed for whatever reason. It turns out that both men and women can naturally have very low levels of circulating HCG in their systems at any time completely unrelated to pregnancy. Less than 5 is usually considered negative. I'm not sure why your clinic is doing betas so late, an earlier one may have given more info as far as if implantation had taken place at all, then failed. I agree that your RE is probably doing a follow-up test to rule out a lab error from the first test.