Strange grey discharge

So I woke up this morning to find a strange discharge stain about 3 cm long on my underwear. It was a pure grey colour. By the time I'd found it it was dry but from the looks of things it was very watery. I've never had this before, although I do sometimes get self-conscious about the odor of my discharge. I know it isn't a sort of STI as I am not nor ever have been sexually active. I am quite clean and wash daily. There is no itching in the area either. My period is due in 3 days.

Is there pain when urinating? Could be bacterial vaginosis, which can smell fishy or can sometimes smell a bit amonia like.   Could just be hormones due to period being due.  Closer to the period it is often white. Unsure what colour this changes to when dry?  But without other symptoms it probably isn’t anything to worry about.  Always best to see gp if you’re worried x 

I'd say it is a change in hormones, as your period is due.