What is the common term for the word "menstruation"?
Can I use cyclus?
In my native language, we normally say it by "monthly guest".
Cyclus isn't a common term. I never heard that word before this question.
If the context is medical you should use menstruate.
Period is the common term. One who is menstruating is said to be on her period. A less direct/somewhat more polite term is time of the month - e.g. It's that time of the month for me ..., Is it that time of the month?
On the rag is a (at least AmE) slang term - not vulgar but nowhere near polite. Often condescending in meaning unless females themselves use the term to express frustration.