My bike was working fine and then one day the chain just started coming off the sprocket. i've moved my back tire back until it was tight again but then i started to lose tension in my pedals. it has been like that for a whiloe and it is really starting to bother me. I've checked my bearings multiple times and did not get any results. I have just a basic mongoose trick bike. Anyone who has a good way to fix it tell me.