What a scarey thing to happen a 20 weeks! Try to keep calm, rest assured that they will keep an eye on you from now and make sure you go straight back if you have any more problems. Keep asking questions so you can fully understand the situation, write them down and go back to you obsteatrician... fear can make the stress worse and it can play out in your body.
I had a similar prob. at 40 weeks after sex, I had no idea about the placenta until they induced me (no labour pains, but lots of blood) and scanned me during labour. I ended up having a CSect. as my cervix wasn't dilating and I haemorraged. I was very afraid and I think at least you have time to get your head around the fact you *may* need a C Section. When you have it sprung on you it's horrible! My bubs was born fine, now running about as a happy 8 year old! I was OK but a bit shaken.
For my 2nd child they did more scans and identified a low lying placenta at 20 weeks. This time the placenta moved as my uterus grew, they checked me at 32 weeks. I was allowed an opportunity to VBAC and that was successsful. I still bled quite a lot but they kept an eye on me. Maybe yours will move too, it depends on where exactly the placenta is right now. Most placentas do "move" by 32 weeks. There are different degrees of low lying placenta, sometimes the placenta even blocks the cervix.
For this bubs I just had my 30 week scan... guess what... another low lying placenta sad They took measurements of bubs to check she is growing well and measured the distance of the placenta from the os (cervical opening) but I have to wait for my antenatal to see what the doc says. I did some research on the net yesterday but I freaked myself out looking at all the worst case scenarios, I find the medical prof. do have to err on the side of caution though... I'm sure they will take you in if they are the slightest bit worried. There are lots of "what ifs" but you can't live your life worrying. But then, don't get complacent and start doing loads of housework!
I assume they said no more nookie, I'm leaving it now lol.
Take care, remember bubs was fine on the scan... eat well, relax and report any further bleeding asap. x