A good afternoon can be followed by a much harder day, and that delay makes post-exertional malaise especially confusing. PEM involves worsening symptoms after exertion that might once have felt ordinary. In ME/CFS, symptoms can worsen hours or days later, so how you feel during an activity may not tell the whole story.
That can make the usual advice to "do a little more each day" unhelpful or harmful. Pacing means working within individual limits, including the demands of thinking, conversation, and emotional strain. It is not a fixed exercise progression or a promise that every crash can be prevented.
Learn from patterns gently
If it is manageable, note a few activities and how you felt afterward, including the following days. You do not need a detailed spreadsheet. A voice note or a couple of words may be enough to discuss with a clinician familiar with PEM.
There is no universal safe heart-rate number or rest interval. A wearable may provide information for some people, but it cannot decide your limits on its own.
Make room for less
Consider splitting a task, asking for assistance, or leaving something undone. Rest can mean reducing sound, light, conversation, or decision-making as well as physical activity. Choose what is tolerable for you.
A crash is not evidence that you lacked discipline. Your available energy may be very limited and difficult to predict. Support should help reduce demands, not make you responsible for controlling every symptom.
Further reading: CDC: Managing post-exertional malaise.