I told my crush a few weeks back that her boyfriend was cheating on her when he wasn't...
Did you know it wasn't true?
I wasn't sure at the time but I suspected.
It is wrong to lie to someone even if it is an accident. You should mind your own business unless you are certain there is cheating going on.
I'm sorry, I honestly didn't know at that time.
That's no problem, mistakes happen!  Just make sure in the future that you are certain before speaking out about something as serious as this.