Your symptoms definately seem to suggest that a nerve in your spine is being irritated, or having pressure put on it. The MRI scan should indicate this for you. Try to be as active as possible, without causing yourself more pain, and be aware of your posture, avoiding heavy lifting, bending your back or twisting it to avoid aggravating your symptoms.Painkillers can help ease the pain, but ask your doctor what is best first.
I am 22 and have just recovered from very similar symptoms (except mine affected the left leg), with pain in the leg, numbness and weakness. If I can offer you any advice, it is this. Do your research once you have a diagnosis and be active in your treatment plan. Discuss all possible treatments and do not leave your doctor until you are happy with your treatment plan. If it doesn't work, go back and try again. There are so many different options to treat your pain, especially if it is a herniated disk (as with mine). It can be disheartening at times but you will get there. :-) good luck, and I hope everything goes well for you.
if the sensation is reduced in your calf and the power in your calf is reduced it is likely that there is some pressure on a nerve in your low back. This could be due to a disc that is bulging or any number of things. I think your physio did the right thing in ordering an MRI etc so that you'll know for sure. I would in the mean time, however, avoid prolonged standing activities and practice positions where your back is in a more "open" posture, for example, lying on your back with your knees and hips bent and hugging your knees towards your chest. This should take some of the pressure off the nerves in your back. Also, if you must stand, try flattening your lower back a bit by drawing in your abdomen. This should also help to take the strain off your back. In bed lie on your back with your knees bent OR on your side with your knees bent and a pillow between your knees OR on your stomach with two pillows under your abdomen. All of these positions would serve to "open" your back a bit and to take the pressure off the nerve and allow it to heal.
How long have you been going to therapy, have you tried injections yet? What are your Doctors telling you, what are they recommending?