I am 20 years old and have had regular periods since I was 16. I've just finished a strenuous exercise regime and a different diet and now I'm 4 weeks late, is it because of the exercise and/or diet?

Your menstruation cycle could very well have been affected by your change in exercise and diet.  It could also be a combination of your lifestyles changes and the fact that you’re only 20 years old.  It certainly is not uncommon for your menstrual cycle to change at your age, especially with added exercise.  It is also not something to worry about.

If you’ve done a pregnancy test and its come back negative you may want to take a second test, or consult your doctor, they will be able to provide a more reliable test.  You want to be sure you’re not pregnant before you concern yourself with anything else.

Here’s a link about irregular menstrual cycles that you may find useful.
http://www.menstruation.com.au/periodpages/irregular.html