Scar tissue that forms at the incision, or surgical site of mastectomy, feels hard.
It will get even harder with time and if you undergo radiotherapy after mastectomy.
Also, it's good to be aware about possible side effects of mastectomy, seroma and hematoma.
Seroma is collection of clear liquid, usually, formed after mastectomy in the surgical site.
Body, often, gradually absorbs this fluid within many days or weeks, but, a knot of calcified tissue sometimes remains (calcified tissue feels hard).

Personally, I think you shouldn't be worried!