Loula the greatest risk of incompetant or incomplete cervix is having had a history of prior (especially multiple) late miscarriages or second trimester labour/birth. Multiple D&Cs could increase your risk but not by much and it also increases the risk of scar tissue which generally will keeps the cervix shut not make it open under pressure. The other thing that can increase your risk is having multiple LLETZ procedures (which is where they take a small bit of cervix to get rid of cancerous or pre-cancerous cells usually caused by HPV) even still the risk of this happening (unless it has before is very low)
As for knowing it might happen usually unless diagnosed on ultrasound that the cervix is shortening and or funnelling then there is no warning it is going to happen until labour starts or membranes rupture. At this stage I would agree with your Obby that the risk is low of this happening. An ultrasound at 15weeks like you said your having and then the usual morphology scan at 18-20weeks should show the cervix been shorter then what is considered normal if this was going to be a problem. Also you can have a shortened cervix at one of these scans and go onto deliver at term so its not always accurate at diagnosing who is going to go into labour and who isn't.
Have they been able to diagnose why you are bleeding or where its coming from?