It doesn’t sound too concerning tbh, I think you may be worrying unnecessarily.
Most likely culprit is an anovulatory cycle I.e. your body either did not have sufficiently high LH surge, or did not respond to it. This is usually because you are tired/slightly unwell. Most women have occasional anovulatory cycles. Instead of having a normal period, you bleed about 5 days early. It can be either lighter than usual or heavier than usual depending on the exact cause, but is usually bright red blood.
Alternatively, it could be an implantation bleed. I assume you are ttc from the fact that you are on this board? However, that is more likely to be pink/brown than bright red. Implantation bleeds (when they occur, not everyone gets them) are about a week before your period is due, much lighter than your usual period and last for a short time.
Neither of those is a cause for concern, so please don’t be worried! One-off weird cycles can happen for all sorts of reasons, if your cycle is usually not problematic it is nothing to worry about. If you are ttc, fingers crossed it is the latter, in which case you would have enough HCG to get a positive test if you wait another 5 days or so. Good luck!
If you continue to have unusual cycles, then that would be the time to see your GP.