I'd reiterate what others have said - that law school will open up your eyes to areas of the law you might not even have thought about to this point. I enjoyed law school - learning the law and studying the law - but really had no clue where I wanted to apply the knowledge until the very end. I decided I didn't want to push paper at a desk, that I wanted to be in court, and that I wanted to work on compelling cases - the nitty gritty, real life stuff. So I gravitated toward criminal and family law and now work exclusively in family law. So when people ask you, it's perfectly ok to say, "We'll see..."
And btw, every lawyer works in "constitutional law." It permeates every area of practice. For example, in my practice I'm concerned with the fundamental right to parent. Bear in mind, however, that most of the constitutional issues you'll encounter have already been addressed and codified under the law, e.g. procedural due process is taken care of in the Code of Civil Procedure which provides for certain steps to ensure that opposing party is noticed and has the opportunity to be heard. Of course, when you say "constitutional law" you probably mean "civil rights," which is simply enforcing personal rights as set forth in the Bill of Rights.