For young females BP of 90/50 may be normal. Anything less than that is abnormal BP. You need to check your BP regularly to see any fluctuations from your normal BP. Also in case of falling BP, you should immediately take ORS mixed water or water mixed with a pinch of salt & 2 teaspoon of
sugar. This may help in those scenarios.
Next Steps
If you are having repeatedly, then you should visit a physician for thorough workup.