Coolant sensor goes off at random times?

[Max777]

Ok, so I checked it today, the coolant is always lost, but it's not a TON of it, the rad always fills up pretty quick, and also the coolant is LOST pretty quick... like, 15 min of driving the thing goes off. At home for lunch, the upper hose was still cold, but then it got hot like it's supposed to. I noticed that my coolant reservouir has dark coolant in it, and there's like an oil looking thing on the surface. Also, system seemed to have quite a bit of pressure in it, so is it possible that the iron cracked or something, and it OVER pressurizing the system? My overflow bottle DID have a bit of extra coolant before the T stat opened, so...

Also, I'm going to go order the neck from Mazda, but is there any other place cheaper, that can get it to me ASAP? Advanced didnt seem to be able to order the part.