At one year post-op, your description is accurate for capsular contracture. Since bleeding and bacteria are the two most common causes of (late) capsular contracture, do you remember any moderate trauma to your left breast at about the time of the tightness, tenderness, and switch from sitting lower to higher than the opposite breast? If so, you might have had some bleeding, which now has developed into the tight, firm, sore capsular contracture. The other cause would be bacteria--can you specifically recall an incident where you may have been ill enough to have bacteria in your blood stream at the same time as a minor bump or bruise to the breast that could allow the bacteremia to enter the space around your implant and stimulate the capsular contracture? The most common scenario where this can occur is with dental work, including cleaning. I have seen several cases of dental work-induced capsular contracture (late, unilateral, 1-3 weeks after dental work). Though I am in a minority of plastic surgeons who do this, we believe that antibiotic prophylaxis prior to dental work is appropriate to minimize the risk of developing capsular contracture from dental work.
Regardless of the cause, it appears as if you do really have capsular contracture, and may well require surgical intervention. If you capsule is early and relatively mild, a course of Singulair or Accolate and Vitamin E may reduce the severity of the capsule enough to avoid surgery, but I have seen this work in only a few cases. Still, if your capsule is mild and early, this may be worth a try!
Otherwise, you will require surgical capsulotomy, capsulectomy, and possibly new implant(s). Certainly the size of your implants leave little room for contracture, but unless you really feel too large, I have to assume that until this left-sided contracture developed, you were happy with your size. Smaller implants in the present pockets may be an "easy" fix, but may not be the best surgical choice. Rely on your surgeon's advice, as you will be returning to him or her if the capsular contracture worsens or recurs.