What are the chances this means twins?

Hey all. I feel like I had a different experience from others who've posted about twins on here. I'm 7 weeks and I've been spotting just a little for the last couple of days. I went in for an ultrasound today and the technician showed me my little Peanut and then we listened to its heartbeat. Thank god. The end... Afterwards, we went in with the doctor. He checked out my ultrasound photos and then he said that there is actually a second sack that could have been the start of twins. He said it looks to be shrinking, so he doesn't think so, but when I asked what our chances were he said, "low, but definitely not zero." He wants us to return for another US in one week to check it out. He said my spotting could have been the demise of the second :( I felt as though I could be having twins and so I wasn't surprised that he saw two, but he didn't mention an embryo and of course the US tech didn't point out or find a second heartbeat. Did anyone ever experience something similar and went on to have twins? Honestly, I'd love to have twins so I'm hoping so! :) thank you!