Confused taxpayer

<p>Last year July I submitted my tax return like I do every single year around that time. I was employed on a contract basis and have been for about 5years so every year when I submit my returns I get a good amount as tax returns. Last year I did the same and for the first time I only got R6000 plus and this didn't sit well with me however I let it go. A month or so later I get a call saying I owe SARS R5000. I was confused and asked the lady how this was even possible since I had just submitted and was given a pay out where does this R5000 debt now come from and I've made it a point that every year I submit. How is it possible that I submit in July and get a pay out from SARS after they have counted my tax returns then next month I owe them R5000?</p>