I Have Been Taking Non Flush Niacin 500 Mg For

I have been taking non flush niacin 500 mg for about a month now. For the past couple of weeks I have noticed my legs are very weak. I feel tired all the time and I have pains in my head. I though it was just sinus problems. I have also started taking magnesium for leg cramps and it has been helping. The only other new medication I have been taking for the past 2-3 weeks is claritin for allergies. What could be causing my legs to feel so weak.