I agree with is_something_wrong: I don't think that anything is wrong with you! 156 bpm does not sound bad at all, as long as your heart isn't going that fast all the time, of course. During my holter monitor test, my heart rate got up to 184 bpm! I will often reach rates of 150-160 during the day, due to things like running to the bus, or panicking, just like is_something_wrong mentions. It would, however, be very useful to know what you were doing when your heart reached that rate. Were you running around, or were you sitting still? How did you feel? Did you feel your heart speeding up? If you can remember, or access, that information, it will be very useful in understanding your situation. Either way, I'm sure that talking to a cardiologist will help, and until then, take it from someone who has a lot of experience with fast heart rates: they can be caused by a variety of different factors, and they are not as dangerous as they seem to be. You're going to be fine. :)
It's not at all unusual that your heart rate is 156 bpm during a day, like when running to the bus, climbing stairs, walking up hills, or it can be caused by anxiety/panic, etc.
It seems like your physician haven't interpreted your Holter monitor in details (which is not so strange, I didn't know anyone but cardiologists interpreted those tests). You should ask your doctor for a copy of the entire test (not just the summary) for your cardiologist to interpret. That will give you the answers you are looking for.
By the way, did you have any symptoms during the test?