20 week scan

I had my 20 scan on Tuesday and they said my placenta was low and so have to have another scan at 36 weeks to see if its moved and if it hasn't then will need a c section for delivery.  they didn't say anything else at all, but I have been researching on-line (not always the best thing to do) and it says about bleeding and maybe best to avoid sex.  Does anyone know is this true and why would I have bleeding?  They didn't say anything at all about that at the scan.  thanks.