Sudden onset, even sudden severe onset, is not uncommon during the 2-5yo developmental period. Interestingly, a more sudden, severe onset is correlated with higher likelihood of spontaneous recovery (aka, it goes away on its own after a "phase"). A stutter that starts as a small thing and builds over time is actually more correlated with long-term stuttering. Onset pattern is just one of many factors for figuring out how to approach stuttering in kids this age, though. If your current SLP is not well-versed in stuttering, I'd definitely recommend at least consulting with one who is. It's very nuanced and complex and works differently than a lot of other speech-related things kids can have going on at this age.