Fluctuating blood pressure with weakness can be due to dehydration, stress, irregular meals, poor sleep, or side effects of medicines, and sometimes it may also be linked to underlying conditions. Sudden drops or rises in pressure can make you feel weak or tired. Try to stay well hydrated, eat regularly and balance meals, and maintain a proper sleep routine. If this keeps happening, or if you feel very weak or dizzy, consult your doctor to determine the exact cause and manage it properly.