How do I pray after surgery when I am menstruating and cannot pour water over my body?

Ghusl (ritual bath) after menstruation is obligatory when one is able. However, if you are unable to perform it due to illness, or if a trustworthy doctor informs you that performing ghusl would cause harm, then tayammum (dry ablution) is sufficient for the period during which using water would result in harm. If you are able to wash part of your body but are excused from washing the rest, then you must wash what you are capable of and perform tayammum for the remainder.