Is this something that just started? I mean, did it ever not do this?
There is such a thing as a "fixed gear" bike in which there is no freewheel on the rear wheel. Much as the name implies, there is simply a "fixed gear" on the rear hub. You pedal forward, the bike goes forward... you pedal backwards, the bike goes backwards... if you're rolling, the cranks keep turning...
If it was originally a freehub where you could back pedal and it suddenly started behaving like this, you may have seized your freewheel. If not maintained and lubed properly, the pawls in a freehub can become locked in place making a freehub behave as though it's a fixed gear bike.
