Regular Working Hours

Is it legal for a company to change and decide my regular working hours as they frequent as they wish?
My regular working hours is weekdays 8:30am to 5:00pm and hourly paid.
But I was sent to business travel recently and they paid hours I worked on Sunday as regular hours; also I worked for about 2 hours on Saturday but they did not count it since they said I have to work 8 hours a day to get an overtime. My point is those days were off my regular hours and on weekends for the past 3 years.
Also do I need to write down the days that I worked on weekends during business travel and submit it to the company? It happened on two of my business travel. I worked on the weekends but the company did not pay me neither overtime pay nor regular pay. The company know, especially the payroll department know (because the gave me pocket money for business expenses) that they sent me on travel but did not issued a check for the hours I work.

Any response is appreciated. Thank you.

There is no law against changing your work schedule.

Have you discussed your concerns with your superiors? What is their response?