My husband has the same results from his BP machine. I believe there is an automatic setting on some machines that will give you the warning of "Irregular Heart Rate" if your pulse rate is less than 55-60 BPM. My husband has 1st degree heart block and his normal resting pulse in the morning is about 45 BPM and these are the times the machine warns him of "irregular heart rate". Later in the day, or other times when his heart rate is over 60 BPM, he does not get the warning. Still, if it concerns you I would check with your doc and tell him/her you are getting those warnings on your BP machine. These machines are not really able to stratify rhythm or heart rate irregularities, so shouldn't be relied on for purpose, but they are reported to be helpful to patients with heart block, AFib and some other conditions to help monitor and alert them when their pulse is irregular or too fast or too slow so they can let their physicians know.
Sometimes the wrist monitors are really quirky about the angle they are held, etc. I'm not sure what kind of monitor you are using, but try using a table top arm monitor, i.e. the old fashioned ones. ;)
Blood pressure machines, being machines, can be wrong, and some, if you move a certain way during the test, can give odd readings. I would take it to your doctor, show him/her what is going on and ask if they can take it with their machine and then take yours again with yours, maybe it is just your machine.