Started period; after lunch, legs began to hurt intensely & were weak while walking. walked around & nearly fainted. stopped after 1.5hrs; blood clot?

Unlikely: If both legs were hurting, it is very unilikely to be a blood clot, particularly at your age and if you are otherwise healthy. Different answer if you are on birth control pills and a smoker, but still unlikely to be a blood clot in both legs. This needs a more comprehensive discussion and evaluation with your physician if it is recurrent.