Enemia! drink a lot of water, and take iron pills! but first get tested for enemia!
Anemia is certainly a likely possibility. There are various kinds of anemia, with iron-deficient anemia being likely in a person at your age. A visit with your PCP, including a complete blood count (CBC) and several related tests to check for iron levels (serum iron, ferritin, etc) and such should help sort out what is going on.
My 11 year old daughter had a problem she bruised very easy and had to see hemotologist and was dx with a condition called IPT it is really something you should go to a doctor for there are some lesser conditions like enemia and more scary conditions so you should get checked out