My cycle fluctuates, also. I had very tender and sore breast this past week and a half and just got my period this am also.
Maybe that it is what it is.
Also, if you were to go to a doctor and did have a lump, they are usually not cancerous if you have a cyst and some pain with it. I just went through all that and ended up being fine after some testing.
Just wanted to mention it in case you find you do have a cyst--most times women are fine and it is common.
Have a great day!