1994 Honda Civic overheating repeatedly and coolant loss

A COUPLE OF MONTHS AGO, I HAD MY TIMING BELT AND WATER PUMP REPLACED. ABOUT A MONTH AFTER HAVING THIS WORK DONE, MY CAR OVERHEATED. I LOOKED UNDER THE HOOD, AND THERE WAS HARDLY ANY COOLANT IN THE CAR. I THOUGHT MAYBE THE TECH JUST FORGOT TO TOP IT OFF, SO I ADDED IT MYSELF AND EVERYTHING WAS FINE. TODAY, ABOUT A MONTH LATER, IT OVERHEATS AGAIN, AND I LOOKED UNDER THE HOOD, AND NO COOLANT AGAIN. I HAVE NEVER HAD THIS PROBLEM BEFORE, BUT THE CAR IS GETTING SOME AGE ON IT. WHAT COULD BE CAUSING THIS? MY OIL LEVEL WAS OK.