What Could Tiredness With Weak Legs Suggest?

Hi doctor, for a few weeks i have been feeling constantly tired and my legs feel really weak as if they cant hold me up, iv also begin to suffer with headaches lately. im 5ft 5 and weigh approx 8st 9, i eat a fairly balanced diet and do quite a active job. i had blood tests in the past but they have always come back clear. just wondering if you have an advice. thanks danielle