I have had a small breast cyst in one of my breasts for years. It was a bit smaller than a small olive, but I had biannual mammos and ultrasounds for years to make sure it was stable. This is the first year I've had a mammo only once (since it appeared to be so stable). I woke up one morning this week to find it was huge... the size of the top of a soda can. It still feels oblong, but with an additional, smaller bump on top. It's very hard. Not squishy at all. It also doesn't hurt. Recently, I had xrays to make sure I didn't have pneumonia. Also, my dose of synthroid increased due to rising TSH levels, and I started the birth control pill about 4 months ago. Could my cyst have become cancerous? Could cancer just pop up so quickly... undetectable one day and huge the next? On the other hand, if it is just a cyst, could any of the recent occurances/changes in my life caused the cyst to grow? I'm 45.