If i have had a rash for more than a week and it is now spreading to other parts of my body could it develop into anaphylaxis? and if so how quickly?

Unlikely: Most cases of anaphylaxis will develop within minutes to hours of exposure to an allergen (food, insect sting, drug). If symptoms have already been present for a week and even if worse, it is low likelihood that they would progress to cardiovascular or respiratory involvement. Either way, if symptoms aren't better, see a dermatologist.