Hidden Twin?

So we went for our first US today. I am  6w5d but we had fertility issues so we get to scan early. At the REs office today, he said that we may have twins because the fetus is so low that it could be hiding another one. He wants us to come back in a week to check for sure and see. We only heard one heartbeat but as many of you know they have to pinpoint the exact spot just to hear it. Has anyone else been told this? Anyone have results? Thanks!