Could it be that my parents assume I smoke because my siblings and friends do?

It could be your friends or siblings. If your parents know that your siblings smoke, they could want you to grow up with a better life. Also, same with your friends. But it could be that they don't trust that you only have done it a few times. You could just look like you do drugs. It might be the way you dress or the way you wear your make-up or anything really. If your grades have gone down or you're getting mood swings, they could be suspicious.