The bulging of the flesh on the inside of the foot next to the arch may be caused by several reasons: 1. It may be caused by an infection here. For this area, it is more common to have some puncture injury here, followed by a bacterial infection here. In this case, there will be a very obvious swelling in this area, or even in the whole foot. In addition, if the patient has had severe tinea pedis, which is a fungal infection of the foot, it may lead to a severe pediculosis in the foot during its onset. This can also induce very obvious swelling here. 2. It may be caused by a tumor here. There are some important blood vessels and nerves on the inner side of the arch, and if there is an occupying tumor lesion inside it, it may also cause this bulge. 3. It may also be caused by skin lesions here. For example, due to the frequent friction and squeezing, callus will be produced, which in turn will cause swelling and pain when pressed. In addition, this area may also be infected by viruses, leading to diseases like corns or toe warts.