What could cause your period to randomly come a week early?

My period came early. I was only on Day 24 of my cycle. Usually, my period comes between day 30 and 32. I've never had it earlier than Day 28. Never. I was really shocked to see some brown spotting starting up this afternoon. Why would my period come almost a week early when it has never done that before?

I don't use female/hormonal birth control. I use condoms for pregnancy prevention. I have never had an early period like this.

Should I be concerned about pregnancy or other health issues?

What could cause your period to randomly come a week early?
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