What Causes Dark Brown Spotting Instead Of Proper Period After HSG Test?

I had an HSG test on day 20 ( i did not have sex during these days), and I should have my period on day 25-28, but on day 25-27, i only have some dark brown spotting, very light. Since i do not have sex this month, I do not have a chance to be pregrant. What's wrong with me?