Is (suffering from) cancer painful?

My (former) neighbour, a thirthyish year old girl/woman, has breast cancer, and although we have had open, straightforward conversations about her being sick, treatment, impact on both mental and physical area, I have untill now not asked a very obvious in my view question:
Is it painful to have cancer?
I am asking specifically about physical pain.
Does anyone know what cancer does to a body?
Since I understand that having (had) cancer is very hard and a very private matter, I understand that one might not want to talk about it on a public website.
In that case and if you would not mind feel free to send me a PM with what you would answer.
Thanks in advance.