Hello everyone! If this is in the wrong place for the thread, please let me know.

So let me start off by saying that I have a 2009 R55 Clubman S.
This issue just started last week, and I will go through everything that happened.

First, the end of last week my check engine light came on (on the speedo), so I went to Autozone to get the code read, in which, it was: "ASE certified Master Technicians have seen this issue on your type of vehicle and the most likely solution is: Replace Engine Coolant Temperature Sensor (ECT)."

I decided to check my engine coolant and look into replacing the sensor myself (which I found the correct sensor and O ring, but not yet bought"
I found that my coolant reservoir was almost empty, so I filled it to the "Max" line. Shortly after, I drove for about 7-10 minutes, stopped, turned the vehicle off for a few minutes, then turned it back on. The check engine light went off, and everything was fine for the rest of the day.

The following day, I was driving home, then the "Engine is running hot" lamp came up on my tachometer in yellow. Unfortunately I had just entered the freeway, so I monitored my speed as best as I could. The lamp turned off less than a minute later.
Then, the same warning lamp came on, this time it was red. I was still on the freeway, and getting really close to home, so I decided to cautiously drive home and monitor my speed, (yes I know, I should have pulled over to let the engine cool down, but I had to get home to go to bed for work within six hours).

The next day, everything was doing just fine (I was only driving to the essential places, which were short drives), the check engine lamp was not on for most of the day. Later in the evening, the "Engine running hot" warning came on once, in addition to the check engine lamp on the speedo.
I have only been driving those short distances since then.

Now onto today. the check engine lamp was still on, so I decided to check the coolant reservoir again, and it was almost empty. I filled it again, the check engine light went off as soon as I started the car. I drove to Autozone to get the code read, the code read the same "P0173B".
After I got everything read at Autozone, I noticed that the reservoir was below the minimum less than 30 minutes after I filled it to max. I filled it again and noticed a gurgling sound, and I watched as the coolant level was getting lower before my eyes, in which I filled it up to max until it stopped lowering.

I have not checked the reservoir since I filled it four hours ago (intermittent driving to and from school, so I am letting it cool down before I check again).
In addition, I do not see any coolant on the concrete below my car.
I do not know what is going on, or what to do about it other than take it to a shop.

What are your thoughts/suggestions?

Much appreciated in advance.