What Causes Lump On The Lower Calf?

I have about a quarter size soft lump on the outside of my lower left calf, on or near what I believe to be the soleus muscle. I also have it on the right leg but there is 2 other smaller lumps on the right side. They re not painful and I know there has been at least one on each leg for years Im just curious if it s part of the muscle or what since it kind of pops out when I flex my foot upwards.