She had an operation on her uterus

A woman may experience something that causes her to bleed
from her vagina, such as an operation on the uterus or close to it. This
falls into two categories: 

1 – If she knows that it is not possible for her to
menstruate after the surgery, such as if she has an operation to remove the
uterus altogether (hysterectomy) or blocking it so that no blood can come
from it. This woman does not come under the rulings on istihaadah
(non-menstrual vaginal bleeding); rather she comes under the rulings on one
who sees some yellowish or brownish discharge or some wetness after her
period ends. So she should not stop praying and fasting, or refuse to let
her husband have intercourse with her, and she does not have to do ghusl in
this case. But when she wants to pray she should wash the blood from herself
and put a piece of cloth or something similar over her private part to
prevent the blood coming out, then she should do wudoo’ and pray. She should
only do wudoo’ after the time for the prayer has begun, if there is a
specific time for the prayer such as the five daily prayers, or when she
wants to offer a naafil prayer. 

2 – If she knows that there is no reason why she should not
menstruate after the operation and it is possible that she may menstruate.
This comes under the ruling of istihaadah and is indicated by the words of
the Prophet (peace and blessings of Allaah be upon him) to Faatimah
bint Hubaysh: “That is a vein and is not menses. When the time of menses
comes, then stop praying.” The words “when the time of menses comes”
indicate that the ruling on istihaadah may apply to one who has a period
that comes and goes. If a woman does not have a period then her bleeding is
regarded as coming from a vein in all cases. End quote.