Hormones, puberty. Of course it is normal, don't worry about it. It happens to all of us! But don't let your feelings get the best of you, if you want to have sex make sure you are 100% ready and sure, be with someone you care for, use protection. Better yet, if you are planning on it, wait until you are older. But if you are just having "feelings" and thoughts, it is still completely normal and a part of growing up.