Could be that your chain is slipping on your free wheel if its a new bike make sure the chain is set up properly or if an older one the chain may have streched.I found to test your free wheel is to support your bike hold the breakes while on it peddle backwards and then go forward with preasure if it slips or "clunks" its a good chance it is your free wheel but on expieriance try you your chain first. Or your chain tentioner make sure its inline if you have one.just looked at your bike kabuki mono same as me its the chain mail me if you want.