from injuries, implantation or withdrawal bleed?

Hi,
Welcome to ehealthforum,
Anal sex usually will not result in pregnancy unless, the semen leak has come in contact with vagina (and if she was during her fertile days at time of intercourse), chances of pregnancy are possible. If she had stopped her regular birth control use and given a full menstrual cycle gap (stopped pill use for a full cycle), or given more than 7 days gap between starting her next scheduled birth control pill pack (after the withdrawal bleeding/period), she will not have the protection. The symptoms that she describes could indicate pregnancy possibility or even side effects of hormonal pill use (it is difficult to distinguish between the symptoms as they can be similar during early pregnancy). The bleeding after masturbation that she experienced while being on pill-free time, could be indicative of her impending withdrawal bleeding. Wait and watch for her periods. Look out for development of any new symptoms if pregnancy is suspected. She should visit her gynecologist for further evaluation and assistance if she does not get her periods within 10 days of expected period dates, or gets abnormal bleeding. If pregnancy gets ruled out and if you are not ready for pregnancy, but intend to remain sexually active, she should be regular with birth control use as instructed. Avoid having unprotected sexual contact if you are not ready for pregnancy. She should stop the use of hormonal contraceptive if pregnancy is suspected, or if pregnancy gets confirmed.
Take care.

Then what caused those excessive discharge mentioned?