It's really wonderful that Butters can detect a migraine coming on and signal that to you! Do you have any theory of what she is detecting? A scent? A change in your movement? A sudden change in your heart rate or blood pressure? A change in your breathing?
Doctors have a theory that when our serotonin levels drop just before a migraine that the dog can pick up on it by scent. I think she does that, but it's also by sight and sound. Anytime I have a hemiplegic, she's more adamant on getting to me and my usual first symptom I have one coming is a droop on the left side of my face, usually my eyebrow or the corner of my mouth. I also start to stutter a little just before one hits, sometimes it's not so noticeable to me or my fiancé, but I think she's seeing and hearing the changes and alerts me.
I've actually lost part of my sight and all of my hearing during the more severe hemiplegics (the night before the 2016 elections, the sheriff's department and EMTs had to fish me out of the mud of my front yard because I lost my sight and fell against my car, she was in the pen and was not happy in the least that there were a bunch of strangers in her yard taking care of mom), so I honestly think she knows that if I don't stop what I'm doing and pay attention, something really bad could happen. I've fallen a lot since my migraine status changed, walked into a lot of door jambs because I can't see them properly and she's been there to witness all of that. I think she remembers and she's trying to keep me safe.