To add to what others have said, this is very common. Because stuttering is largely mental, a lot of disfluencies are situational in nature. I can read out load by myself and be 100% fluent. Reading the same thing in front of 100 people while it is being transcribed, timed, and I'm representing the general public? Not so fluent. Some things you could try (skip if you know these) are, ease slowly into the first sound, slow your rate down a bit (or a lot, no one will think it's weird but you, trust me), or stutter as easily as you can. This all take practice (a lot, really) but they may help. Good luck.