There are various reasons for this. The most common reason this is happening is due to poor hygiene. Without proper brushing and flossing the gums become inflamed and will grow up to or over your braces. This causes discomfort. Certain medications or pregnancycould cause excessive gum growth as well. Also, poor positioning of braces could cause some gingival or gum-swelling inflammation. To deal with this issue proper brushing is essential. Brush at least twice per day for at least 2-3 minutes. Also floss and use a Waterpik.