Dreams are the mind's way of processing information and thoughts, including subconcious issues it is trying to resolve.
Unfortunately, this can mean nightmares.
Have you had recently, or got now, any stressful situations at home or work? Any bad relationships, or any worries at all, however insignificant? If so, you may need to chat with your GP about this, who may refer you, or give you some temporary medication to help you over this 'bump' in your sleep patterns.
The good news is, nightmares rarely last for long - the mind will heal itself and move on, because it is extremely good at protecting itself - even from itself at times!
Do see your GP, and don' be embarassed, this is far more common than you might imagine, but your doctor can put your mind at rest - literally.
Good luck.