Would a sudden increase in population, increase inflation? Let's say, for the sake of the argument, that the US population grows from 320 millions people to 420 millions overnight, wouldn't that increase inflation?

What if in reaction to this, the monetary mass is left untouched?

Thanks
It would lead to demand pull inflation as the goods in the said economy would not match the demand. But this depends on the time frame and how the producers in the said economy react.
Let me give you an example of immigration. Let's say large numbers of people from a country leave their country for the said country. Ignoring all social issues let's see what would happen. On one hand inflation would increase as the immigrants would cause high demand for goods and services which would drive the prices up. On the other hand, the immigrants would bring in supply of labour which  lead to a drop in real wages and decrease the prices of goods thereby reducing prices. So an increase in population may or may not lead to inflation