Thanks for all responses!!

I stay hydrated, so I can exclude that.
It really can have something to do with a digestion, I sometimes have problems. Though this pain feels more gynae related and that should be OK for me - I have yearly checks and no problem reported. I'll ask about this anyway.

What is bad about this pain is that it's too late to stop / slow down when it appears... I'm trying to say that it doesn't develop, it just appears in all its 'beauty'. That is a reason why I often hold back in training, as afraid of the pain starting again. At least I know what triggers it! (the heavy hills)