grammar - What does ます形 + そうになった mean?

While reading an article, I came across this sentence (the article is describing people's reaction to a movie):

若い女性は「泣きそうになりました。すばらしかったです」と話していました。

A young girl said, "I started crying. It was amazing."

I was under the impression that the ます形＋そう is used to guess or describe based on appearance and is usually translated as "seems" or "looks". It seems odd to me that someone would describe herself as having a "crying" appearance rather than simply stating that she was crying. Is this a different usage of そう that I am unfamiliar with? Is she perhaps stating that she actually did not cry and only looked like she was about to?

Finally, just to solidify my understanding, would the meaning change significantly if instead of 泣きそうになりました she said something like 泣きました or 泣いてしまいました?

Answer

「Verb in 連用形{れんようけい} (continuative form) + そうになる」

can only mean "to be about to (verb)", "to come close to (verb + ing)", etc. I would strongly suggest that one learn this as a set phrase.

You were obviously thinking of the meaning of 「そう」 in general when it is not followed by 「になる」.

Thus,

「泣{な}きそうになりました」

does not really mean "I started crying." It means:

"I was about to cry." or "I came close to crying."

In other words, she did not really cry.

Finally, just to solidify my understanding, would the meaning change significantly if instead of 泣きそうになりました she said something like 泣きました or 泣いてしまいました?

Yes, of course. That would mean she actually cried.