what does spotting on the 18th day of cycle indicate?

Women can experience spotting at any time during their cycle, and it is not an indicator of any medical condition or pregnancy.

Spotting is extremely common, and is called "breakthrough bleeding" (bleeding in-between periods). It is more common for women who use hormonal contraception. Spotting can also occur after intercourse (even the next day), that is noticeable on toilet paper after using the bathroom (in other words: not a large quantity of blood...literally a few spots on the toilet paper or your underwear).

Spotting can also happen if a women becomes pregnant, known as "implantation bleeding", but this is usually unnoticed by many women.