Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services if arm stiffness or loss of control begins suddenly with any of the following:
These can be signs of a stroke.
Do not drive yourself to hospital.
Emergency assessment is also appropriate after a serious head, neck, or back injury if you develop new weakness, numbness, breathing difficulty, or loss of bladder or bowel control.
Seek same-day medical advice if established spasticity becomes much worse and you also have fever, confusion, urinary symptoms, severe unexplained pain, or red and hot skin.
An infection, wound, or another treatable problem may be triggering the change.
Book a routine appointment if tightness is new, gradually worsening, or affecting movement, hygiene, sleep, work, or caregiving.
Arrange review if the fingers are pressing into the palm, the skin is breaking down, or the arm is becoming harder to move.
Every sign above is quoted from the sofidex article on Upper limb spascity. If you are not sure, err upward — it is always reasonable to be checked.