I have a lump on the left side of my neck where the shoulder and the neck meet. what is it?!?

Right above my colarbone I noticed a small lump, its closer to my should than to my head. It is like right where the neck and shoulder meet above the collarbone . I think it is on the muscles & when I play with it cause it worries me it hurts & it feels like my muscles are getting sore also. It is hard and will move side to side. It did not hurt until i started fiddling around with it. The only reason I noticed it was because I was playing around with my seatbelt when driving one day. Ive only had it for about 3 days now. I am hoping it is a bad muscle knot or maybe a cyst. I'm going to make a doctors appointment tomorrow but till then I'll worry about it. someone at my work told me if it moves it nothing to worry about but if it doesn't then you should worry