I have had a really sore chest for about 6 days now, i am on holiday so slightly worried!

I have no cough or flem and it feels sore down the middle of my chest. Sometimes it hurts more than others, mostly so on the nightime, when i take a deep breathe it doesnt hurt that much more. I think it started just after i walked for an hour or so in the rain, i don't know if that had anything to do with it!