Was it a heart rate monitor on the actual machine? Those in-built ones can be extremely inaccurate. I know once I jumped on (and was only warming up) and it says my heart rate was 206 BPM. Conversely, when I was in a full sprint, it has said my heart rate is only 58 bpm (when it would be around 180-190).
Because you are 38, your maximum predicted heart rate would be about 182 bpm (220 - age). But getting your heart rate to 180bpm at 38 years of age is extremely high and would require you to really push to your limits.
Seeing as though you didn't feel overly exerted, it is a very good possibility that the heart rate monitor was not accurate. However, if it was accurate in it's reading, then I would strongly suggest for you to go to your GP and make sure that you are in good enough medical condition to perform this exercise regime.
A good idea is to take your pulse from the right side of your neck to see how accurate the reading is. Count how many beats you feel in a 10 second period and multiply by 6 for your bpm. This will give you a rough idea of your heart rate.
