Given it can take up to two weeks for an infection to become symptomatic, you may want to keep an eye on your mum anyway, if you saw her at any time whilst you were ill.
As to flu vs COVID19, flu has a mortality rate of approximately 0.1% for your stated 650,000 deaths.
If COVID19 comes out at 1% (and that's a figure that's widely considered likely) that's substantially higher.
It's also way harder to prevent infections given there is no jab for it, so the base figure of infections could come out higher than flu as well.
This is why self-isolation is a thing, and not for flu, because the high risk groups (elderly, immuno comprimised, etc) cannot protect themselves from people spreading COVID19 like they can with flu. It's not about you or I (assuming basic healthiness), it's about the population at large.