Have you ever had problems with oil consumption in a Ford Fiesta

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I have a Ford Fiesta 1999 and currently I have to fill up the oil every 3 weeks. Does anyone have any suggestions as to what is wrong with my car? Could it be a leak? Or Engine problems? Wrong oil? Mechanics are so expensive that if it is possibly something simple that is worng with it at least I'll know I'm not being ripped off by the mechanic.....