Heart rate monitors are generally terrific tools for monitoring exercise intensity and providing immediate feedback to enhance training; but they vary in level of quality, and they don’t work well for everyone. Models that feature chest-strap sensors are typically most accurate.
An actual maximal heart rate is only obtained by completing a maximal exercise test in a clinical setting. If you somehow determined your maximal heart rate based on data from your heart rate monitor, something’s not right. Predicted maximal heart rate is commonly calculated by subtracting age from 220; in your case, that would equal 172 beats per minute (bpm) so a reading of 236 bpm for your maximal heart rate doesn’t make sense.
Your heart rate monitor could be picking up artifact — capturing signals from movement or other types of interference and interpreting them as heartbeats. In addition, a variety of medical conditions can cause an elevated or irregular heart rate, which could result in an abnormal reading on your monitor. If this is the case, you could use the Rating of Perceived Exertion (RPE) scale to subjectively monitor exercise intensity after getting your health care provider’s advice regarding an appropriate level of exercise.
I strongly recommend that you visit your health care provider for an evaluation before beginning or continuing your exercise program. Once you have a clear picture of your exercise abilities and any limitations, a certified fitness professional with the appropriate level of training and experience can help you establish a safe and effective fitness program to help you reach your goals.