Before I had children, I had a couple of cysts that were very scary to find. About a month after I stopped breastfeeding my first child, I developed a lump. I had it checked by having it aspriated. It was another cyst. About a month after I stopped nursing my second child, yet another lump appeared. My yearly exam soon came and my doctor checked out the new lump. It had all the same characteristics of the other lumps (feels round, seems to move back and forth easily and at times, extremely painful). He and I both felt it was another cyst. He said to keep an eye on it and if it changed, come back. It hasn't and it's still there. He said that when you stop nursing, your body goes through hormonal changes and that sometimes triggers changes in breast tissue. Every time I find a new lump, I go to have it checked out. I would definately suggest the same for you to have done as soon as possible.
Hang in there!