What Does Thickening Feel Like???

Hi,
i am 33 years old. For the last 2 months, i've felt a minor pain (like a stitch) in my left breast, or it could be in my pec? It's beside my armpit/upper breast.
I had a clinical breast exam and my doc said she felt nothing wrong, no lumps or fibrocystic changes at all, lymph nodes not swollen.
I don't know what this little pain is?
If I feel around too much, I can make it worse and then for a few days, i'll notice the whole area and my ribs feels tender.
Now, in that same breast , the lower area, it feels like a thickening but i'm not sure.
Its like a large rubbery area that moves back and forth. Maybe this is normal breast tissue?
Breasts seem to have areas of thickening.
So, what kind of thickening should we be concerned about? Does anyone know??