Mystery stains on inserts

Ok, so I change Kiara this morning, take the inserts out of my minki and there's a great big blue stain which looks like a colour run on one of the inserts that was definitely not there before.
The inserts were a couple of homemade hemp one, a white bamboo fleece one (home made too),a cotton terry one (also homemade) a greenkids booster, and a pink mf that's been used many times. It's the second time it happens. I use a marker pen when I draw on fabricinstead of taylor chalk but it's a special one for fabric.

Any ideas????

Could it be a stain from something else that was in the wash? Some fabrics take up colour run a lot easier than others. We tend to use colour catchers if we think there is something in the wash that is likely to stain clothes, and they work really well.
Is your fabric pen the type that you mark it on the fabric, and it disappears within a day (or an hour when the pen is getting old like mine)?

Somebody elsewhere said it can be caused by calpol - which she has had recently as she's got a nasty cold.
I had no idea it could be that.