When you say that "lately" you've been afraid to go swimming, I take this to mean that not that long ago, you weren't afraid. So, my question would be did something happen recently that has made you afraid?
Did you nearly drown or believe you were going to drown? That has happened to me and it took me a very long time to work up the nerve to go back in the water.
For you to have the reactions you describe, all of which point to classic anxiety, it sounds like something rather traumatic happened to trigger this fear.
If you have always enjoyed swimming and suddenly became afraid, I don't believe this is just about being a teenager.
Not knowing what brought this fear on, I would suggest letting your parents know and deciding if seeing your doctor to rule out a possible physical cause being at the root of this new found fear. If there is nothing physically wrong, then it may be a psychological issue and talking with a therapist could help you discover the cause and beat it.
I wish you the best.