Did I ever tell you how I got on birth control and it ruined my life?
There are times when birth control can have negative side effects.  Is that what has happened to you?
I don't want to explain it.
You don't have to tell me anything if it's too personal to you.  However, I would advise that if you are having a bad experience with birth control, you could talk to a doctor about other options that you might have.
Can we change the subject?
Of course, but please just bring up the subject with someone that you are comfortable talking to, as you do have many other options.