Unique I just checked, he went 11 days after Jess. He hardly hung around for weeks 🤷🏻♀️
I think quite a few here have said about Michael still being into Amber being the reason for his anger. Could be they're right and now he's seen her with someone else the jealousy he's feeling means he has to admit to himself he still wants her.
Either that or the old "I don't want you but I don't want anyone else to have you". Personally, I might be leaning towards this. He likes the thought Amber is moping over him, he likes the ego-stroke and doesn't like the thought of her moving on.
Third option could be it's suddenly dawned on him he has to be coupled up to stay and knows Amber is his only chance.