Work and fertility appts

My doctor only does ultrasound 7:30a-9:30a. We are doing IUI this cycle and it could happen any day, I know it will be last minute. I work 6a-2:30p. I told my boss I have some medical stuff coming up and I might have to call out. She said "thank you. Let me know when the appt is so we can figure it out accordingly". Only a certain # of people are allowed off on certain days at my job. I feel like this is more than work and it's honestly stressing me out that I will lose my job if I call out to many times. And I do have PTO I can use. I just feel stressed when it comes to work and appts