Testicle Pain, like strain.

I am 14 years old and I have been having this problem for about 4 years I think. So, sometimes randomly (maybe not). My testicles start to hurt, it is like a straining feeling. Also it seems to be in the left one more and also the part of the tube. The pain isn't unbearable but it will hurt more when i move it. The pain has never lasted longer than half a day and usually goes when I sit down to rest it. My testicles look fine and hasn't got any lumps that people talk about in other posts. Is this normal as in does this happen in puberty??? 

Thanks in Advance!