Of course there is no way of telling for sure,but your doctor will let you know,
after reading your complete Ultrasound report,if the
lump needs to be further investigated by having additional tests or having a biopsy to make sure what this lump represents.
I don't think you should be overly concerned about it,because not all hard lumps that form inside the breasts are cancerous and since yours is mobile,it's a good sign.