Which would be the difference between "hon simmar pa mandag" and "hon simmar pa mandagar" (sorry I have no diacritics). Does the one mean that in the first case she goes swimming this one Monday and in the second one she goes swimming every Monday?
The system shows you what it thinks is the closest correct solution to what you wrote. Using "is swimming" is not an uncommon construction, so I think it's fine, even if you won't find it in the grammar books. :)