but I still want to know what my rights are
Anna, honesty no offence intended but the reality is, if I were your employer and you raised this issue with me, I would be seriously questioning your commitment to your role and your place in the company. (regardless of what your rights might be in this situation)
I don't know what job you do but generally speaking I don't understand overtime outside of McDonalds, Subway and other service sector jobs where people are paid minimum wage, by the hour and every hour counts. If you're employed in a professional role and you care about career development you should put in more hours than those around you, get in to the office before your boss and leave afterwards and don't complain it openly to colleagues or HR.
I know that doesn't necessarily scream efficiency but employers are more often than not looking for people who can commit themselves fully to the role.
Some friendly advice, that's all.
I would doubt there's anything positive that would come of raising it as an issue with HR