If it has been running low and decreasing slowly over time, then you might have a slow leak in your radiator. Usually, if you have a major leak it will all come out at once. The ar many different products available at your local auto supply store that can be poured into your radiator to seal a slow leak.

Can you see the leakage? If so, is the leak in the front near the radiator or slightly back under the engine? Check your hoses. Ensure that your clamps are tight and there are no punctures or breaks in your hoses. Check your radiator for cracks. Make sure the valve on the bottom of your radiator is closed tightly. Your leak could also be a sign that your water pump going bad. Sometimes when water pumps are on there way out they leak radiator fluid.

I've had this happen a few times over the years with various cars. Most times it was just relatively normal operation. A few months and a few thousand miles may knock you down enough to require a top off. It is possible that you had a hose get a small gash/pinprick in one of your hoses or a clamp loosen up and weep coolant out. This is relatively minor and can be repaired inexpensively. Be warned that it isn't always easy though, depending on the vehicle. Replacing the upper hose on my '89 Aerostar proved to me that there is no God and disproved the theory of "Intelligent design". Then there are the ever-popular head gasket and it's cousins, the warped head and the cracked block. Usually you'll notice these by looking at your exhaust and checking for EXCESSIVE water vapor. (A little water is expected as a normal byproduct of combustion.) While this can get quite expensive to fix ($500-$2000), the good news is that at least the vehicle will usually be able to be driven to the garage. Usually. And that assumes it's the head gasket. Warped heads are not always repairable and cracked blocks... forget it, just swap the engine. Lastly are water pump seals going. Those can be a trickle or a torrent, but will usually leave a puddle under your vehicle. If you're good, you can lose 1 gallon a minute and paint the windshield of the car behind you with ethylene glycol. Needless to say, this won't last long and neither will your engine.Adding a gallon every 5 miles for an 18-mile trip sucks and SERIOUSLY shortens your engine's lifespan. I got lucky; I made it home. Assuming the engine doesn't completely die on you horrifically, a water pump is fairly inexpensive to replace. However it doesn't take long for a blown water pump to kill an engine; a head gasket leak usually lasts a lot longer before dying.

its alswo possible your thermostat is stuck open i would change the thermostat after flushing the system the add new coolant

it could be 1.blown head gasket:cheak oil and antifreeze if they are mixed or not, 2.bubbling in radiator,smell of collent:can be leak somewhere in radiator. take UR car for UV ray test 3.There might me some problem that can be hard to figure out even mech have those problems.