Low lying placenta

I've just had my 20 week scan and they found that my placenta is only 8mm away from my cervix. I've got to go back in a couple of weeks anyway as baby wasn't behaving so they couldn't get one measurement! They are going to reassess the placenta at that time. They said not to worry and that it should move up during the pregnancy. They told me it was nothing to worry about and the only impact could be bleeding in the last few weeks of pregnancy and possibly c section. That was all they said. Nothing about any impact on the pregnancy now. But now I've read around it I can see other people have been told to not have sex and be really careful with what you lift etc. Even that I could have heavy bleeding soon, does that mean my baby is at risk?? What have other people been told who've had the same thing?

Thank you