Could a herniated disc / bulging disk be affecting thecal sac?

Yes: However, this pressure on the thecal sack is most probably insignificant. I would not be worried by that report that usually you will see the radiologist review.

Disc herniation: Depend on the size and the type of the herniation, it might affect or even compress the thecal sac.

Maybe: If a disc herniation is large enough, it may compress the thecal sac.