Hi there and thanks so much for posting this question!
First congratulations with the practice of such an incredible sport of Rugby, one of the most physically and mentally demanding sports.
From your description it looks you had a more serious injury than you previously thought. A strain in the quadriceps is not a normal injury, you need to ***** where is this strain, is it in the top closer to the psoas or closer to the knee.
A quadriceps strain is a tear in one of the quadriceps muscles at the front of the thigh. It can range from a mild discomfort to a full blown tear of most of the muscle.
You may have what is more commonly described as quadriceps tear degree 1
Grade 1 thigh strain http://www.sportsinjuryclinic.net/sport-injuries/thigh-pain/quadriceps-strain
Symptoms of a grade 1 quadriceps thigh strain are not always serious enough to stop training at the time of injury. A twinge may be felt in the thigh and a general feeling of tightness. The athlete may feel mild discomfort on walking and running might be difficult. There is unlikely to be swelling. A lump or area of spasm at the site of injury may be felt.
Anatomy of the Quadriceps Muscle
The muscles of the thigh help to provide the strength needed for locomotion, and provide the stability for the knee joint. The thigh muscles are divided up into compartments and the muscle group that occupies the majority of this region is the quadriceps muscle, which forms the mass located at the front and side of the thigh.
From your description try to stay away of any anerobic activities including sprinting, i know that in rugby is impossible. Take few days off till it feels better.
My suggestion to start it would be to do the qaudricep and hamstring stretch followed by performing gradual sprints and see how it feels
Here is an example
To build quadricep strength try to work asymetrically on the leg is more injured and do this exercise http://www.youtube.com/watch?v=DsSGnpzoTzs&feature=c4-overview&list=UUNO4_pf5rxOelDCdsF-gd-Q
If the pain persists it means you had a much bigger strain as previously thought and you may need to rest completely and go to your sports doctor and monitor if there has been a larger tissue affected since those ones dont recover that fast.
Hope this helps and let me know how you feel!