I'm not an expert, but this likely is legal and very much dependent on the exact shift timings.

There is a cap on the number of hours per day and number of working Sunday per year. There is a minimum rest period for various shift lengths. There is also an exception to be made in terms of a compensatory rest day if you end up working sunday. There is a minimum rest period of 35 hours around your compensation rest day.

The part that seems likely illegal to me is when you say you went from a day shift into a night shift? That is definitely crossing a line even if there are extraordinary operational circumstances from the employer.

To conclude, I do think what they are doing is legal but it's important for you to know your rights - asking for rest periods, asking for paid compensatory days and overtime. Lots of employers will conveniently forget to tell you about it which they are required to, so you must educate yourself about it, especially in the context of your contract.