Probably your head gasket is on the way out. The coolant is probably leaking into the cylinders and coming out the exhaust. I had this problem, cost me £250, got the water pump done at the same time and totalled £330. Just keep an eye on your coolant level to assess the state of the gasket, faster coolant drains, the more urgent it will be to replace. Leave it until you can afford it but dont leave it too long or you will damage the cylinder head. Theyll have to do the (or they should) the cambelt at the same time.
After it was all done on mine, the coolant stayed put from 85000 miles when gasket was done, to 108000 when I sold it (thursday).
Hope this helps, but bear in kind Im no mechanic, I might be wrong, it just sounds very similar to mine.
Chris.