Dental Fillings do consist of 50% mercury and can potentially cause mercury poisoning in humans so if you've had dental fillings and start feeling sick as a result you should see your doctor and get tested for mercury poisoning and if so you can have the filling removed by the dentist.
Some people get dental fillings and never experience a problem but some people do get affected by the mercury contained in the dental fillings and have to have them removed otherwise they get mercury poisoning which can be deadly.
A friend of mine had dental fillings put in as a kid and then later on in life she started feeling very sick and she got tested and turns out she was getting poisoned by the mercury in the dental fillings.
After she had the dental fillings removed she never experienced the sickness that she was filling when she had mercury poisoning.
So yes sometimes dental fillings may cause mercury poisoning while other times it may not.