....eek. Mistake :(
I had a HSG to test if my tubes are clear (for fertility reasons) and it failed despite 4 attempts (and apparently the women who did it was the most experienced HSG person in the local Women's hospital).
I'm not sure exactly why it failed - I was in some pain after the 4 attempts and didn't think to ask 
Anyway I had an ultrasound the next day and it showed my cervix was 'bulky' - the sonographer said this could be the reason my HSG failed and refered me back to my consultant for urgent appointment.
Does anyone have any ideas about this? What could be going on? I wish I hadn't googled now :(