Sounds like you have an oil leak, possibly at the piston ring(s) if you cannot spot oil leaking out of the engine itself. Best to have a trusted mechanic check your car's engine. If you dont have one, you can bring it to a shop like SPEEDYFIX (along Shaw) or SPEEDLAB (along Quezon Ave).

Is your exhaust smokey?

Anyway, from your description, you'll probably need to have it overhauled (if the problem is from the piston rings).