Blood clot on placenta

Just went to the doctor for my 19 week check up/anatomy scan. Everything with baby is wonderful! Measuring right along with due date and healthy as can be. However, the doctor said that I have a blood clot on the placenta, so she said that's something we're going to monitor in about 6 weeks. She said she has seen them before and usually they don't cause any problems, but we're going to monitor with another ultrasound in 5-6 weeks.

Has anyone else experienced this? If so is there anything I can do to help make sure it doesn't get larger or cause any issues? Also- what could have caused this? I am not overweight (125 lbs pregrego and 5'5"), very active (marathon runner), and healthy eater!