There are several reasons for the delay in menses. In a reproductive age woman, it is generally related to pregnancy. Typical over the counter pregnancy tests become positive at 2 weeks gestation or one week after implantation. When the pregnancy test is negative, other conditions that can cause a delay in menses include: anovulatory cycle where your body did not release an egg at the appropriate time. Conditions that can be associated with anovulation are emotional stress, weight fluctuation, hormone imbalance such as hyper or hypo thyroidism, or pituitary causes such as a tumor (adenoma) that can increase your body's prolactin level. These can be tested through your blood. You can request for a blood test to check for pregnancy (Bhcg) as well as basic hormone testing through your women's care provider.
Breast cancer is one of the most common cancer types affecting women and an increasing number of younger women are affected with this condition. Breast conservation treatment with lumpectomy generally require radiation therapy and/or chemotherapy depending on the stage of breast cancer.
In general, chemotherapy can cause a temporary decrease in ovarian function which leads to amenorrhea (skipping periods) and hot flashes. With time, periods can resume and many women are able to achieve pregnancy.
The more important questions is, "Was the original cancer sensitive to estrogen and/or progesterone (Estrogen receptor, progesterone receptor status) sensitive?" Because high levels of estrogen and progesterone occur during pregnancy, some studies indicate that any potential breast cancer cells can be stimulated during the pregnancy which means that if there is recurrent breast cancer, going through a pregnancy may worsen the health outcomes. One of the ways a woman might avoid this is to have a surrogate carry the pregnancy.
So to answer your question, breast cancer survivors can get pregnant without assistance if they have regular menses after treatment. Once they are pregnant, they can stay pregnant until term unless they are in the middle of treatment when they get pregnant. Breastfeeding is possible with lumpectomy although some of the ducts may be scarred from surgery. A woman with complete mastectomy in one breast will not be able to breastfeed from that breast. I hope this is helpful.