Wa'alaikumassalam! Jangan risau sangat, low-lying placenta at 20 weeks is quite common lah, a few of my mum friends had the same thing and it resolved on its own by third trimester sebab placenta akan bergerak naik je as the uterus grows. Definitely mention it to your doctor at the next scan so they can monitor, tapi chances besar it's nothing to worry about, they'll just keep an eye on it with ultrasounds along the way. Make sure you rest enough, avoid heavy lifting, and no strenuous activity je, then go for your follow-up scans as scheduled to check if it's moved up. Kau ok je!