It may be doing too much too soon. Your tendons may not be ready if you have not done long rides this season and are starting to do long rides. As previously mentioned bike fit may be an issue, but I would first gradually build up milage. You may also want to ice your knees after your rides and keep the rides at low intensity until you are sure your knees are fully recovered from last weekend. You definitely do not want this problem to linger for the rest of the Summer.