In the context that you gave me, "μου" is not related at all with "είχε λάβει".
It goes with the previous phrase "το σώμα" and it is a possessive pronoun meaning "my".
"το σώμα μου" literally means "my body"
The verb "λαμβάνω" (in Past Tense: έλαβα), in ancient Greek has various meanings. It means "receive" , "gain" and it can also mean "take over". Cavafy uses modern Greek in a special way, including many idiomatic elements and words and endings of ancient Greek. So, in this context, the verb "έλαβε" means "took over.
The whole phrase "το σώμα μου είχε λάβει ο Έρως" if you translate it word to word, means:
"Love had taken over my body".