SI or sacrum pain

Hi,

62m, active, very active: cycling, bodyweight exercises and pilates, some dumbell work for a rotator cuff issue (fixed), surfing.

Surfed last week 5 times. Rode bike a fair amount. stretched between sessions. In recent months started pilates on mat 2x/week with no issues except sore hip flexors sometimes, and loving it.

In the last few days, a dull general pain around sacrum, sometimes a small shooting pain when I stand up but not always. Possibly SI joint pain from what I've read. No treatment yet.

I read this subreddit and thought it would be a good place to get some feedback.

My older carcass may just need gradual efforts to become somewhat surf fit. I've been at it a few years. I think I'm suffering from hip inflexibility..( and likely weakish hip flexors..)so getting leg under body to pop up is like a herculean effort for me (think burpee but with a torso twist so feet land perpendicular to the starting body axis). Maybe all the recent efforts added up to an SI issue? Any ideas? Thanks in advance!