Nope, not unrealistic at all! Your period is triggered by a drop in hormones (progesterone). If your hormones drop enough for any reason, bam, your period starts. Diet CAN cause a drop in hormones that fast.
Based on what you said about potential implantation bleeding, I'll assume you're not on birth control and had already ovulated this month. Had you started the whole 30 earlier in your cycle, your period likely wouldn't have appeared so soon.
The first time I changed my diet (not paleo at the time - insulin resistance diet), my period started 3 days later and I didn't have a fraction of the normal pain due to endometriosis. It happened that fast. Not the same as what you experienced, but it was a testament to a jump start of the hormone changes!