(non pregnancy) causes of HCG increase?

[HotI]

Given that the levels only went from 3 to 12 is it possible that it is a lab error? As they are usually looking at levels in hundreds or thousands, just wondering how accurate they are in the levels below 20?

ETA:
I found this on the SydPath website:

To be confident that a pregnancy is present a serum hCG concentration greater than 25 U/L is desirable. If a concentration between 10 and 25 U/L is obtained on a pregnancy test, then a further sample in 1 - 2 days is suggested for confirmation.

hope you get some answers,

Kate