Need advice

I have noticed a hard red lump on my dogs leg yesterday morning and not sure what it is 

I'm not sure either, but there are lots of possibilities.  The top layer of skin may have been scraped away to the layer below, perhaps (depending on the depth) make the lesion an ulceration or erosion.  It is hard to tell how it has come about;  whether by a splinter or trauma such as a scrape or burn, or an infected fly-bite, or self-trauma (perhaps nibbling a hole in response to allergy - is it within nose- reach?)  I hear that you describe it is a lump and would love to get my fingers round it in order to assess it properly, as it does look quite superficial in the picture.  However, if it's a) a lump, or b) if it is being bitten or c) if it does not disappear quickly, this may merit a quick check from your vet.  They may want to triage over the phone in order to figure out how quickly to book your in.

.....book *you* - and your dog - in.   With apologies.