I doubt it caused this issue, calcium carbonate from these kind of tablets are often used for rats with kidney degeneration as a phosphate binder and they tolerate a good amount without issue. Longer term it could be a problem if they were getting one daily but as a one off his system wont have been able to absorb it that fast.
Rats can get nose bleeds as such, much like we can, they can also have a large flow of porphryin if something irritates there nose enough. It may be worth making sure he hasnt got his paws on any other medication though, as some could cause more problems. For now i would keep an eye on him and if he starts looking spiky, fluffy or lathargic get him to the vets ASAP