Mike: So you two are finally gonna get hitched. who popped the question? was it you? Matt, or did you propose to him, Allison?
Matt: I decided to be the romantic one, I took Allison to the park where we had our five-year anniversary picnic, and asked her there...
Matt: she was in a bad mood that day, I almost lost my nerve, I thought she was gonna turn me down
Allison: He's right... I almost left him in the park. I had a terrible migraine. Stress at work. And he'd been acting so strange, I guess getting ready for the big question.
There is no subject because it's redundant. Alison could say: "I guess he'd been getting ready for the big question". But that has been said at the start of the sentence so there is no necessity to repeat the same thing.
She is talking with Mat and Mike now. And she is giving her opinion now that's why she uses Present Tense