The phrase "I'm on the pill" typically refers to a person using oral contraceptive pills (birth control pills) to prevent pregnancy. There is nothing inherently wrong with using this phrase, and it's a common and straightforward way for someone to communicate that they are using birth control.
However, it's essential to consider the context in which you use this phrase. Conversations about birth control can sometimes be sensitive or private, so be mindful of who you are speaking to and the setting in which you are discussing this topic. Additionally, in some cultures or contexts, discussing contraception may be considered inappropriate or uncomfortable.
If you have any concerns about discussing birth control or other personal matters, it's entirely acceptable to keep such information private. Remember that personal decisions about contraception and reproductive health are entirely up to the individual, and there is no obligation to share this information with others if you are not comfortable doing so.