It can be some lymph
node which is increasing in the size or can be some muscular lump in the region.
As per your description, the area which you mentioned is having lot of lymph nodes, but swelling in lymph nodes appear when the area which is draining into it is having some sort of infection or some other problem, which is causing the lymph node to increase in the size.
Apart from that the swelling can be some sort of problem in the muscle or some vessle in that region. It is very difficult to make any diagnosis without examining the swelling and having a proper history.
I would suggest you to have consultation with doctor regarding this problem as this is increasing in size and is important to make a diagnosis.
Thanks and take care.