Growling, aggressively .'im very worried so any help would be great.

I have a gorgeous 17 month old bitch called Lou Lou who is amazing in every way other than 1 big problem.
I let lou lou on the sofa with myself and my family, I have 5 children ages from 5-17 .
Lou lou loves everyone and is very friendly with everyone but now the problem!
If lou is on sofa and one of the children or my partner walks towards sofa she will growl sometimes it’s just a rumble and other times it’s quite bad or she will be on my bed and my partner comes up to bed she will do this growling? I don’t understand why at all!!
I was telling her off every time she done this but then I read I shouldn’t as she is warning and if I keep telling her off the warning could disappear and she could bite.
I really don’t understand first why she does this and second what I should do so any help please.
I did have lou lou done a couple of weeks ago