Pain in my right knee... Please help!?!?!?

okay so about a month ago I noticed some slight pain in my right knee. it wasn't much it just hurt when I bent it.The pain was in the back of my right knee, on the right side and it was deep in my knee. Over the past month it's gotten worse and worse and at the begging of the week it was really bothering me. I do play softball. Monday it was bothering me and Tuesday we had a game and so I wore one of my sisters old compression braces and it really didn't bother me that much during the game except when we were in the dug out. Wednesday it was hurting. Thursday we had another game and this time i didn't wear the brace because I felt it didn't really help at all. thursday during the game it really bothered me. and the pain also spread across to the left side of my knee. (still in the back). Friday I woke up and it didn't feel to bad but now the pain is all the way around, even the front part of my knee hurts. The weird thing that I don't get is that it really doesn't bother me when I run or anything, but when I stop and sit down it hurts, it's like a really bad ache deep in my knee. and it throbs. but it's not excruciating. bending my knee really far hurts, and straitening my leg also hurts. there is no clicking but i noticed it has been popping a lot more lately. It also gets pretty stiff. like it takes me awhile to move it after it's been in a spot for awhile. Our school trainer thinks its my meniscus and thinks i should get it checked. but if it's my meniscus wouldn't' it hurt worse when I ran too? Plus wouldn't it be alot more painful? It just aches and throbs when i sit. I need help. should i go get it checked?