My recommendation basically is to ask your health care provider what they would consider a low for you. Blood pressure ideally is supposed to be between 100 to 120 systolic
(top number) and 70 to 80 diastolic (bottom number). In reality, blood pressure readings should be taken in context, looking at the individual, their age, and what medical conditions they have. For instance, I would be usually be concerned if a 95 year old patient's blood pressure was 90/50 (this is considered low), as I would be worried they might get dizzy when they stand up (and be a risk for a fall). But, if that patient normally had blood pressures that low, was OK and that is their "normal" number, then it would be alright.
Think about yourself, and what you normally run. If you are normally 140/80, and you get a result like 90/50, that would be low for you. Low numbers are especially concerning if you feel symptoms from your blood pressure being low, like dizziness, fatigue, or fainting (often called syncope). If this is happening to you, your blood pressure may be too low. Let your health care provider know if you are concerned, as there are many health conditions that can cause low blood pressure. They may want to have you come to the office for a visit and blood pressure check.