Limits of the JRB Nest Under Quilt
First of all, Blackbishop is correct. The Nest is advertised as a 3 season quilt. Most people can use it by itself into the low 30s. There have been reports from people who needed additional insulation in the 40s (personally, I think these are installation issues) and there are reports from some people who have successfully ventured into the teens (I think there was generally some form of augmentation (pads) used in these cases). Bottom line - you were probably beyond the quilt's capability at 20 degrees.
Blackbishop is also correct about checking correct installation. With the hammock empty, when you stick your hand between the hammock and UQ there should be the slightest gap. With your palm lightly touching the hammock your knuckles should be brushing the UQ. The absolute best method to check installation is with two people. Put one in the hammock while the other checks the loft of the quilt and adjusts suspension cord until the quilt is just barely touching the hammock and is fully lofting below.