i'm assuming you didn't put the cog and lockring on there.
somepeople put a fixed cog on the freewheel side, then put a bottom bracket lockring on top of that.
It's ineffective because both thread off in the same direction.

to be safe you might want to take it to your LBS and have them check it out. , or do some research and do it yourself.
i'd suggest the latter, it's always good when you can learn something new to maintain your bike.