Toilet overflowing without flushing
It's definitely a backup from a source higher up than the toilet.  It could still be on the same floor of the house/building. i.e. The washing machine drainpipe is likely higher than the toilet.

The backup can be clear if the source is something like a shower, dishwasher, washing machine.

Since you are in an apartment, you should first contact your landlord before trying to clear the blockage yourself.