No, isn’t a sin to study on Sunday. You have given honor to God at Mass and you clearly have a reverence for the Lord’s day.
From the** Catechism of the Catholic Church**
2185 On Sundays and other holy days of obligation, the faithful are to refrain from engaging in work or activities that hinder the worship owed to God, the joy proper to the Lord’s Day, the performance of the works of mercy, and the appropriate relaxation of mind and body.
You will have worshipped God…just make sure you have some relaxation…and in studying with your friends you
are helping each other (works of mercy).
2187 …Traditional activities (sport, restaurants, etc.), and social necessities (public services, etc.), require some people to work on Sundays, but everyone should still take care to set aside sufficient time for leisure.
This extract from the second paragraph, 2187, tells you that the Church does acknowledge that work is sometimes necessary for some people, and so long as they honor God and go to Mass, and have some rest, and show some kindness to others, they are fulfilling the Sunday obligation.
God bless you and your fellow students in your studies and in your lives.