Periods suddenly a lot closer together... help!

Hey everyone. So, my periods have always been about like clockwork, never more than a day or so off of the 28 day cycle. But suddenly, in about the past four months, my periods are a lot closer together. Now, my cycle is only about 20-21 days instead of the typical 28. I’m really not noticing any other abnormalities, though. My symptoms are the same for every period, and my flow is the same as well. Also, I’m nowhere near menopause or even perimenopause — I’m only in my mid-20s! I did just finish getting the HPV vaccine shots, but that’s the only change I can think of.

Can anyone help me explain this or provide any insight?

I have an appointment with my gynecologist next week, but I’m kind of freaking out a bit.