Low coolant warning light and code P00B7

My car’s check engine light came up and right around the same time my A/C stopped working. I checked the light and it gave me a P00B7 code which meant that the coolant was low. I checked the coolant level and it was almost full. As I kept driving, the low coolant warning light came on so I took my car to my mechanic. They ended up replacing the thermostat and it seemed fine for about 5 days but then the same thing happened. I noticed the A/C would turn off and on randomly throughout the day but usually not for that long. Once the check engine light and low coolant light came on though, it stayed off until a couple of hours later. I also noticed the A/C turned off when I would stop the car at a stop light and turn back on when I sped up. I wanted to see if anyone had any experience with this before I take it back to my mechanic. Thanks.