Pressure in my goggles after ~25 min

Hello, I am relatively new to swimming and have been getting into triathlons. I have been starting build up my endurance and working on longer swims. Once I start to get about 25-30 min in, I begin to get lots of pressure on my goggles to the point where I have to stop and re-adjust them. Being new to swimming, I don't know if this is common, if this is a user error, or a problem with the goggles I have. If anyone can shed some light on what might be going on that would be fantastic. Thanks!