Yup, go to the doctor. Among other things, hyperextension can tear your PCL which may not immediately prevent you from walking or even running, but needs attention nonetheless. How does it feel if you sit down with your knee at a right angle, your muscles relaxed, and your foot planted on the ground; then yank back on the top of your shin? If it feels uncomfortable at all, especially compared to the other leg, you might have a problem.
Who knows though? It could be anything.