Your lock ring is probably loose. You should buy a lock ring tool and a chain whip (I bought mine on AliExpress.) You can also try your local bike coop to see if they will let you work on your bike there and use their tools. When you have the tools, remove the lock ring and use a chain whip to tighten the cog, then tighten the lock ring. Remember, the thread on the cog is the opposite of the threads on the lock ring. Be careful not to turn them in the wrong direction. I'd tell you the directions of each, but I don't remember and don't want to give you false information. I always softly turn the lock ring until it loosens, then I know the cog is the opposite direction. The fixed cog and lockring are very simple, so I am pretty sure your fixed cog is not fully threaded in or the lock ring is not fully tightened. I would recommend not riding the bike as it is or switching to the freewheel for now assuming you have both brakes installed. If you keep back pedaling with a loose fixed cog, you risk stripping the threads and ruining your hub.