Please help I am so so scared I have cancer..?!?

I am so scared that I have cancer. I started with a pain in my right lower stomach in January, which I thought was appendicitis but turned out not to be. I feel sick a lot, although not actually sick. My thigh aches constantly and my lower back, and my right nipple seems to tingle a lot, I also had pain in my shoulder blade which doctor thought was shingles. Sometimes feel like I'm struggling to breathe. I've now found a lump on the back of my neck, slightly to the right. just below the hairline. It is hard and round. I have cancer don't I?!? I'm 21.