There's a whole bunch of reasons why you might have a little blood on the sheets, and it's usually not a huge deal unless it becomes a regular thing. "You should be concerned if you're bleeding after sex every time you have intercourse, and it seems like the bleeding is heavier over time," Dr. Young says. This could be a sign that you have a serious medical condition, including cervical cancer or an STI. "It is always wise to rule out a serious health condition before you assume it's something easy," she says. Make a mental note about how much you're bleeding, even if you just notice it once, so you can figure out if it's actually A Thing.