I have the same thing, been bothering me on and off all this year but not so much that I've been tempted to shell out cash to see a physio.
I get it immediately after a Parkrun or tempo, also had it 10 miles into a HM in March but ran through it. Sometimes it will travel right round to the top of the hamstring at the back and sometimes catches me out when walking and suddenly twisting round.
Not sure what it is, looking through the hip threads on here it could be anything from a strain to a replacement job!
If it were me I would keep moving but drop the intensity. You may find it makes you a bit fresher and faster on the day.