Sometimes things interfere with heartrate monitors, so your heart rate may not be that high.
Do you feel like you are really stressed physially, like you are at your limit?
Do you have a wireless cycle computer?
Were you with your friend with a heartrate monitor?
Were you riding near power lines.
It is always OK to see a doctor or other health professional with questions like this. Ask your parents if their health insurance provides a call in consultation with a nurse. Sometimes this will save the expense of a doctor's visit.
I am not a medical person, but if you aren't having other symptoms, and you were my child, I would probably assume it is the heart rate monitor... and try to eliminate the possibility of a false reading before getting concerned.
Also, heart rates are somewhat genetic with fitness coming into play as well, so yours doesn't have to be the same as your friend... But I would expect you to feel like you were pushing prety hard if your heartrate was really 175.