Having sex on your period does not usually make you truly bleed more overall. However, it can make bleeding seem heavier for a short time, especially during or after sex.
This may happen because sexual arousal and orgasm can cause the uterus to contract. These contractions may push out menstrual blood that was already in the uterus, so more blood may come out at once instead of gradually over the next few hours.
Period sex can also be messier because menstrual blood is already present, and movement can make it more noticeable. This does not necessarily mean your period is heavier or that you are losing more blood than usual.
You should pay attention to bleeding that is unusually heavy, lasts longer than normal, happens after sex when you are not on your period, or comes with pain, fever, bad odor, or unusual discharge. Avoid self-medicating for abnormal bleeding, and see a gynecologist or primary care clinician if bleeding feels different from your normal pattern.