It means to get pregnant but in a bad way, either the woman didn't want to get pregnant or it was an accident. People would say it about someone when they think that she shouldn't be pregnant. So if it was a young girl who people found out was pregnant they might say "Did you hear that Mary got knocked up?". It's a very impolite thing to say.

I'm not sure without more context, but to me 'knocked up' means 'pregnant'. Does that fit the context? "She went over to the island for the holidays, and came back knocked up".

It's very informal, not something to say in the company of strangers

"knocked up" is when a female is pregnant