It could well happen that we have a fresh General Election in a few weeks time. It could also happen that we have a fresh referendum. Why is one of those possibilities seen as undemocratic, but the other is not?

I've yet to read any comprehensible reason why deciding to have a fresh referendum on something about which there has already been a referendum is in any way more undemocratic than deciding to have a fresh general election which wasn't timetabled, as happened in 2017, regardless of the fact that when people voted in 2015 it was for a five year parliament.

Before the fixed term Parliament act, of course, a Prime Minister could decide to have a General Election any time they wanted. Now it's harder, but it can still happen any time, if the appropriate procedures are carried out.