Re: Lump in breast...suspicious...
I would definitely get it checked out - I actually just went through this myself. If it is related in some way to milk back-up, it could develop into mastitis, and if it isn't, you'll probably want to have an ultrasound on it to confirm what it is.
What happened for me was that after confirming it wasn't mastitis, I was sent in for an ultrasound and mammogram. Those found calcification, which is caused by many things, but can be a sign of cancer, so I had a core biopsy (which sounds bad but doesn't hurt and is a simple procedure). Everything was fine.
Really, cancer is probably the least likely thing for this to be, but it's better to have it checked out and put your mind at rest.