I feel like crying but I dont know why?

Okay so I think I feel this way because Im going away for 2 weeks for the summer to my birthplace & my big sister's not coming along(she's 22).And when I thought about her getting hurt or something horrible happening to her while Im away on vacation I feel tears prickling behind my eyes.Then again it could be my period symptoms.Help? I love my sister alot & everytime I think bout something bad happening to her I feel like crying.