Do I have the right to change my locks?

My landlord gave someone (plumber) permission to come into my house without first notifying me, I was next door at a friends house (left my door unlocked) and when I came back last night i was locked out and had to spend $50 on a locksmith! I was TERRIFIED b/c I thought someone was inside my house!! The top lock was locked also (and the only way you can lock the top lock is if you have a key or from the inside)..So I was under the impression that someone was inside, I didn;t know what to think! I wasn't expecting anyone. Am I able to change my locks TO PREVENT THIS FROM HAPPENING AGAIN?? I'm pissed off b/c she didn't have the decency to let me know someone was going to be at my house, I don't feel comfortable and it's violating my privacy.