Or know what causes it?

Just more of my body weirdness...bear with me.
Honestly, I never noticed this until I was married/sexually active/trying to conceive. Several months before getting pregnant with my son, I would notice that several days before AF was due, I would start getting cramps in my inner thighs and down my left leg...sometimes all the way down my leg to my foot.
The cycle I got pregnant with my son I'm pretty sure I didn't have this (just crapmping in my inner thighs, but not down my leg.) Since I got my post partum period and have been cycling again, I've been having this again..last month starting at 5DPO and this month starting at 3DPO.
I always take this as a sign of AF coming/not being pregnant, but honestly...how can my body even know at 3DPO that my eggo didn't get fertilized? I mean, I doubt I'm pregnant but it's just annoying to have my hopes dashed already at 3DPO. Can't my silly body let me have hope for a little while...say 10 days?

Sometimes I wonder if it's a sign of low progesterone. Ha, who knows...just wanted to throw this out there and see if anyone else ever experiences this?