Those can be soft markers for Down's yes. Did your doctor order a blood test and give you results yet?
False positives aren't unusual in screening testing for Down's, though, and most soft markers are yellow flags, not a given that anything is wrong. Plenty of mothers get false alarms. 2mm fluid measurement isn't really high, if I remember right, I think that's well within normal range.
If your doctor gives you bad odds and subsequent scans aren't helpful, there are other tests you can consider. Amniocentesis is extremely accurate. You can also try chronic villus or percutaneous umbilical cord sampling. These tests are considered invasive and there is a minimal risk of miscarriage associated with them, but it may be worth the risk to you for peace of mind.
http://www.cdadc.com/ds/downsyndromepregnancysymptoms.htm
i have heard yes it can be a sign but also that lots of woman when they went back for their next scan had no problems at all and the baby was completely fine so dont stress to much it wont help anything hun :)