Hi justy - I noticed this in my left eye (not brow) for the first time in March. It can be a sign of some serious stuff (stroke, for a start!) so it is important to get it checked out. In particular, if your pupils are different sizes when it's happening it's important.
I fixed an appointment with my optician (it was anyway time for my annual eye test) and asked if it meant anything serious. He said it was likely due to at least one of the ten viral infections I'd had up to March in quick succession (colds) and that having examined me, he couldn't see anything to worry about. I asked if I needed to see my GP and he said that my GP would have sent me to an optician.
That said, I noticed a recurrence lately and I haven't had any more viral stuff recently so I think I will anyway mention it to my GP just in case.
Better safe than sorry, I think! Having ME for a long time sometimes means we are at risk of attributing all our new symptoms to it (a mistake our doctors can make as well, I think). I'm glad you've started this thread - I'd like to hear about others' experiences with this too!