I got my first Pfizer shot on July 28th and got my period 2 days early on Aug. 11th. It's not unusual for me at all as my period comes early quite often.
I normally don't experience any cramping or pain unless I eat dairy or drink fizzy drinks during the week leading to my period. When my period started, there was no cramping initially. I went for a run roughly an hour after my period started and I can say, it triggered and exacerbated cramps. By the time I got home, I had some real cramping. However, I can't say the cramping is 100% related to the vaccine as I drank quite a lot of fizzy drinks the days prior, so I'd say it's probably related to that as well from past experience. I had a Nurofen as soon as I got home and the cramping disappeared within 15 minutes. So, the cramping lasted probably 2 hours in total.
Flow wise, my period seemed a bit lighter than normal though. My period lasts four days exactly, but on day 4, the flow ceased completely whereas normally, day 4 I'd still bleed but the flow would be far lighter than the 3 previous days. The color was also a bit lighter than normal.
From a PMS standpoint, my symptoms were surprisingly OK. Bloating would normally be my biggest issue, but not this month. My anxiety was a bit more intense than normal and I was exhausted, but it's not unusual for me.
I'm meant to be getting the second shot on Sunday, so there's a good gap between the second shot and my next period. Hopefully, it won't impact it.