My codes are going minus on me

For the past couple of months on my dashboard where the digital miles driven and the odometer are located there's been three digit number appearing behind a tool symbol that started at about 800 that every week or so it changes to a lower number by a hundred and then a week or two later another hundred to where its now at -800. I've been trying to get to auto parts store for a code read. In the meantime does anybody know why this is happening and do I prepare for the worse? I can't afford a whole lot. I just recently came out of being homeless after two years. I'm still unemployed but managing financially for basic necessities selling things I pick up for free on local market places so I can't afford to much but even worse if I should end up loosing my source in freedom personal, financially, etc. It's all I have my 23 yr old son gave it to me three weeks after I moved to a housing program. 1999 Mercedes c230. Please if someone can let me know what's going on i'd appreciate it. Please if its serious brake it to me gently. Thank you very much.