The advice given here is pretty solid. Be sure to rest and recover first and then try to systematically eliminate each variable.
Adjust one thing at a time and revert if it doesn't help. Clearly something on the bike isn't playing nice.

In the end, it could be a combination of problems, such as too high/low saddle height, the pedal platform, and/or your form.

Does any pain come about when in a different position on your bike (e.g. standing, forward, etc.)?
Are you knees pedaling inward? Your body is compensating for something wrong.