I looked on so many sites, so although this thread is old, I thought I'd reply so others see it.
It is probably fine. Nappies change all the time, but if baby is showing signs of dehydration (e.g a dipped fontanelle), the poo smells sour or is diarrhoea, seek advice- it can't hurt, and you wont be treated as though you're neurotic. Small babies can dip quickly, so they will be very understanding.
Last week, I took my 5 week old to my GP after a week of sour green poo and 3 days of diarrhoea. I was told it was colic and to keep her hydrated.
After two days (and a number of calls to my GP & HV)I took her to A&E and she was admitted to hospital instantly. We were there for seven days while they carried out tests and given fluids. It seems she had a stomach virus that left her with an intolerance to lactose or milk protein.
Her weight dropped dramatically during that time and I am so glad that I sought a second opinion.
It was awful, but has been so easily solved with an allergy free diet (Nutramigen) and a milk free diet for me while I am breast feeding (less easy!)
From experience, don't delay, seek advice. You are not being a pain or over protective; you're being a good mum!
Best wishes x