Is my fear that my parents will prevent me in the future from increasing my supererogatory fasting, and my concealing my current fasting from them out of sincerity and to avoid their pity, considered ill-will and disobedience towards them?

Congratulations to you on fasting, especially during the first nine days of Dhul-Hijjah. Be sure to stay with your parents, for in that lies their dutifulness and brings joy to them. Your fear that they might forbid you from voluntary fasting out of compassion for you is not a sign of ill-conjecture on your part, especially if these were mere thoughts that you did not utter. For the blameworthy type of conjecture is that which settles in the heart and is spoken aloud, whereas fleeting thoughts that do not settle are not held against one.