Deficiency of vitamin C can cause swelling in gums and they can bleed also . 
The reasons of bleeding gums can be the deficiency of vitamin K, bioflavonoid,  vitamin A , calcium, folic acid and niacin. Tension, smoking, malnutrition,  steroid, medicines of sediment, imbalance of hormones and carelessness towards cleaning the mouth can make the situation worse.
To prevent it eat orange and lemon and eat nutritious food. and take care of cleanliness of mouth