Good afternoon. If the pulse is 90 beats per minute at rest and this is associated with clinical complaints (shortness of breath, increased fatigue, etc.), then it is definitely worth contacting a doctor who will offer an examination algorithm. This can be both the individual characteristics of your rhythm, and manifestations of other diseases (anemia, thyroid pathology, chronic inflammation), ignoring which can lead to various complications.
Good afternoon, at the age of 27, as well as at any other age, the heart rate depends on many factors. The presence of arrhythmia can be both normal and pathology, depending on what kind of arrhythmia it is. To understand whether everything is in order with your heart or other organ systems, it is advisable for you to at least make an ECG to determine the basic parameters of your rhythm and to possibly identify your arrhythmia. But on a conventional ECG, arrhythmia is not always possible to catch if it is not constant. To detect arrhythmias, there is Holter ECG monitoring, when a person with an ECG recording device walks for a day or more, depending on the doctor's recommendation. So, in general, I recommend that you, dear author of the question, make an ECG first and seek the advice of a specialist.