what did I do to my calf?

You tore part of your calf muscle. Lucky it was in the middle of the muscle and not at the tendon insertion site. The muscle will take a couple of weeks minimum to heal. While it is healing, I recommend not doing hard stretches, but continue to try walk normally. Ice the area for the next couple of days to keep swelling down. After about 48 hours, you can switch to heat to increase the circulation and help speed healing. In either case (ice or heat) only put it on for 20 minutes at a time, then off for at least 40 minutes.

If the area swells significantly, or if you notice numbness in your heel or other parts of your foot, contact your doctor immediately. (Sometimes a muscle will swell inside the area and puts pressure on things that cannot tolerate the pressure, and permanent damage can result.)