For years I've been getting this. When I go for a run sometimes after a mile I get period like cramps even though I'm not on a period. I just have to walk them off and they go after 10 mins or so then I'm fine again but I usually just head home. Today was bad though again just after a mile I started getting like spasms (like you do on a bad period) and felt faint because of it. It happens anywhere from mid cycle up to a period this is the window where I often get cramps triggered by running. I'm fine just after a period up to mid cycle I can run all I want no cramps.
I saw the gp years ago because of it and he just said 'some women get pms'. I've got some mefenamic acid capsules but surely I can't take one every time I go for a run in case it cramps..
By the way, the cramps are just triggered by running. If I don't exercise I'm fine I don't get them (unless I'm on a period as normal).