Over the last six months my tooth have begun to decay at an alarmingly fast pace. what can be causing this. could it be systemic?

Get examined: Remember that decay is a disease. There can be many causes of this problem ranging from lack or good oral hygiene to poor diet and infections. A systemic problem which is breaking down your immune response is possible. I suggest a check up from your dentist and your Family Doctor.

Acid: Acid causes the breakdown of tooth enamel, either from bacteria or your diet. Why have you waited six months to have the tooth restored? Now you still need it fixed, only with a deeper and larger restoration.

Tooth decay: Tooth decay is multifactorial and typically progressive without restoration! This is the "nature of the beast" so to speak. Here is a good summary of the causes as a refresher. http://www.webmd.com/oral-health/tc/tooth-decay-topic-overview Sugar, acids, lack of flouride, not brushing and flossing, not seeing a dentist regularly are all on the list. Get in to the dentist quick!

Decay: Tooth decay is progressive infection. Time is your enemy, the longer you wait the more it will spread, the less your treatment options, the more expensive treatment will be.. Please make sure you see a Dentist, and the sooner the better.