What could this be? im nervous....Please answer!?

I am a 15 year old and perfectly healthy. I have had a lump on the right side of my neck that is not very large and is movable, it does not hurt at all and never bothers me. I also have a lump on the left side of my neck that is about pea size and never causes pain, no bother to me and is movable. I also have a lump on the back of my neck kind of right under where the skull bone, it also causes no pain, no bother, and is movable. I have been to the doctors and they know they are there and have never said anything about them. My brother has these sort of lumps on his neck as well and it is just fatty tissue. Is mine also just fatty tissue? I dont think they are lymph nodes because they NEVER go away and they never change size, they have been there for as long as I can remember. If they are lymph nodes why would they never go away and why would they be movable? If they are lymph nodes could they still be there due to my medical past and have just never gone away and wont go away? when I was young i had bronchitis at 6 weeks, pneumonia three times before I was 3 years old, ear infections about every other month that would cause my ears to bleed, and now I have extremely mild asthma. Could any of those things have caused these? Thanks for your help and answers! They are greatly appreciated! I know if they ever do change to go to the Dr. asap.