It's everyone's worst nightmare. You spill a deep secret to a trusted confidante only to discover later that your secret hasn't stayed safe and is being spread all over time. Some people spill juicy secrets as a way to gain popularity or to intimidate others. However, this is not a mature way to act - it betrays the secret-holder and is generally very hurtful. Unfortunately when other people hear your secret they may not stand up for you simply because they are scared of being the next target. Maybe your mutual friend has the goods on them too?
First try to figure out if your secret has actually been told. Don't underestimate the power of paranoia. Try not to jump to quick conclusions and pay attention to your friends' behavior. Just because someone is acting strangely around you, that doesn't automatically mean that they know your secret. They might just be a strange person or feel uncomfortable around you for an unrelated reason. Consider just being truthful. Carrying around secrets is hard work - maybe the truth isn't as bad as you think. Remember that most people are not as judgmental as you might imagine. Somebody's worst secret is another person's no big deal. If that doesn't ring true, then try to choose any future confidantes more carefully.
Remember that not liking someone does not give you the right to share their secrets with others. Try to find some true friends who will stick by you no matter what and will keep your secrets safe. If you figure out who has spilled the beans, consider approaching them. Calmly say how you feel in an assertive way - avoid yelling. You don't even have to wait for a response - just walk away after you've said your peace.
Having a secret spilled is a deeply stressful experience. Don't forget to care for yourself during this difficult time. Try to do activities that increase your confidence and general positivity. Reconnect with your inner strength and try to gain support from the friends that you can trust. You will come out stronger on the other side.