Make sure the UQ is snugged up against your back/butt. And you may need to have someone look to make sure there are no openings on the ends- while you are in it- which will allow cold air to rush in under even a tiny gap.
Could you tell if you were cold over all or on top, or did you specifically feel cold on your back or butt? Which, if you did, can make you cold all over also. But if you felt a cold back, that will tell you a lot about what you need to do.
Is you bag down or synthetic? Older synthetic bags are famous for losing some loft, though it can happen with any older bag. Are you normally warm in this bag at these temps when sleeping on the ground? If so, more evidence pointing to an UQ out of adjustment.
Big possibility: when I first started using a bag as a TQ, I had quite a few cold nights where I became instantly warm just by getting in the synthetic bag and zipping it up and using the hood/neck collar. This was a learning curve for me, overcoming a tendency towards drafts around the neck and shoulder, or really anywhere along the length of a quilt. I do fine these days, but any drafte must be 100% dealt with if you hope to approach the temp rating of a sleeping bag, TQ or UQ.
Next: head insulation is a must. Especially when going from sleeping in a mummy bag to using that bag as a TQ. When you bypass that bags thick, draft free hood/neck collar, you give up a huge hunk of it's rated warmth.
For I consider that the sufferings of this present time are not worthy to be compared with the glory that is to be revealed to us....that the creation itself also will be set free from its slavery to corruption into the freedom of the glory of the children of God. For we know that the whole creation groans and suffers the pains of childbirth together until now.