When i lie down flat on my back, the outer portion of my breasts sink in, leaving me with large indentations. is this a problem or just lack of structure following 2 pregnancies and weight loss?

See a doctor: any changes in the shape of the breast at any position at any condition will warrant a physical evaluation by a physician so please do not ignore it and have it checked

Volume Loss: Most women lose breast volume subsequent to pregnancy and breastfeeding. This is most likely responsible for the "indentations". Make sure however there are no breast masses causing these indentations.