What you are describing sounds like a brief episode of dizziness or light-headedness on standing, which can happen when blood pressure temporarily drops after changing position. Your usual blood pressure of around 100/60 is on the lower side but can be normal if you otherwise feel well.
For now:
Get up slowly—sit for a few seconds before standing.
Keep yourself adequately hydrated.
Avoid standing suddenly after prolonged sitting or lying down.
If it happens again, sit or lie down immediately so you don't fall.
Eat regular meals and avoid prolonged fasting.
If possible, check your blood pressure and pulse while lying/sitting and again after standing.
Next Steps
Please seek urgent medical attention if you actually faint, have chest pain, difficulty breathing, severe/persistent headache, palpitations, weakness/numbness, confusion, bleeding, or repeated episodes.