Signed up to multiple email subscriptions

A few days ago I started getting thousands of emails. Someone used my email address to sign up to newsletters and subscriptions. How does something like this happen? I would get around 20 per minute for hours, from all over the world in all different languages. I have spent the last few days marking them as spam (doesn't work in most cases) and unsubscribing. This is incredibly annoying.Is there any way to stop this from happening again? I changed my password but I had never heard of an attack like this before