way too soon, there wasn't much time to get to know who Jennifer was. so it just felt rushed

First of all, there was no character development for Jennifer. All we know is that she's a English teacher. Derek, on the other hand we know a lot about. And one of those things is that he has trust issues. The fact that he met Jennifer for a couple times and performs intercourse with her is totally out of his character. I smell another Kate situation.

I picked the wrong button sigh but hooking up first thing is not a Derek thing to do

I agree with @simplymeee although I don't really smell another Kate situation. Derek has trust issues though and he started trusting Jennifer way too easily.

Ditto. I agree with Simplymeee and Modernfan.

You know! there was a big reason for how they grew close to fast!... He saved her from werewolves trying to kill her... and then she finds him bleeding to death... There is nothing strange about how fast they did it

He trusted her so easily