Pilot initiated the BDN RNAV Z approach over HERBS but did not realize that the HERBS transition had an altitude constraint and so began his descent early but did not turn to FOMBO. Confused when he realized his errors; he executed a go around; reprogrammed the approach and started again.

Narrative

[I was] cleared for RNAV Y Runway 16 approach to BDN via transition from HERBS fix; cross HERBS at 7;000. I believed I had selected the correct approach and transition on the GPS (Garmin AT GX-60) but I was unaware that the RNAV Z Runway 16 also had a transition from HERBS. Crossing HERBS I began the descent from 7;000 to 6;100 as published in the transition to DSD. As I was descending; the GPS announced a turn to FOMBO was necessary. This didn't seem right; but at first I thought there was a fix between HERBS and DSD I had missed. The light came on and I flipped the page of the approach book to the RNAV Z and saw that there was a transition from HERBS to FOMBO on that approach. I immediately began a climb back to 7;000 and a turn toward DSD and tried to formulate a plan. Trying to brief the new approach quickly; I saw that the transition altitude from HERBS to FOMBO was 7;000. I decided that it would be less workload to continue with the approach that was actually loaded than to try to re-enter the correct approach; single-pilot; in IMC and light turbulence. I turned back toward FOMBO and requested to be re-cleared for the RNAV Z. This was made a little more difficult because the Controller didn't seem to initially understand what I was requesting or why. Looking back; the fact the there was more than one RNAV approach to 16; each with multiple transitions which have fixes in common; should have alerted me to be extra careful in selecting the approach.