12.5 year old Chihuahua

Hello, I have a senior chihuahua (12.5 years old) and he has a heart murmur, I believe grade 2. 3 days ago I let him outside to use the restroom before bed. He did his business while it’s 15 °F out. Walked up the flight of stairs to come back inside and once inside he started coughing, and fell to the floor. His body got all stiff and after about 20 seconds was able to regain himself. I just thought he got lightheaded and dizzy. Then he did it again tonight.   I will be bringing him to the vet first thing Monday.