Chest hurts in the middle of my breast.... But when I lay down it don't hurt as bad... Got me worried. Not sure what to do.

Muscular. It sounds like a muscular type pain based on your age, gender, and the positional quality. Have your tried simple analgesics like naprosyn (naproxen) or ibuprofen? Massage may also help.
Ice and ibuprofen. This is most likely costochondritis or inflammation of the sternum. Applying ice for about 10 minutes several time a day and taking Ibuprofen may help. It is not likely cardiac(heart) in origin when it goes away with lying down and due to your young age. However, if associated with other symptoms (radiating to arms, sweating or shortness of breath), or does not improve in a few days, see you dr.
Please see your doc. Or obgyn or go to the er depends where it hurts which side and how bad may be breast related or something in the chest or chest wall.