We're not sure if it's the hormones or just fatigue at having your friend always in the spotlight, but Ron ends up at odds with Harry for a good portion of this film. Here's what happens:
The Goblet of Teen Conflict
When Harry's name somehow makes it into the Goblet of Fire, Ron (for some reason) gets really annoyed at Harry, seems to forget everything he knows to be true about his best bud, and assumes that Harry somehow made this all happen. And he's really peeved.
Of course, since Harry didn't have anything to do with his name making it into the Goblet of Fire, he's pretty offended by Ron's attitude. So, they're totally at odds: Ron has decided that Harry is an attention-seeking cheat, and Harry is (naturally) offended that Ron could ever think that about him.
The result? The two boys aren't speaking as Harry faces one of the most difficult challenges he's faced so far. Of course, we the viewers are heartbroken—we love the friendship between Ron and Harry.
Ron is so mad, he has Hermione to communicate with Harry for him:
HERMIONE: Ronald would like me to tell you that Seamus told him that Dean was told by Parvati that Hagrid's looking for you.
Oh, man. That's taking the silent treatment to a ridiculous new level.
They make up relatively quickly, though. We suppose that watching Harry almost getting killed in the first task of the Triwizard Tournament kind of snaps Ron out of whatever weird paranoid thoughts he was having about Harry and his motivations:
RON: I reckon you have to be barking mad to put your own name in the Goblet of Fire.
HARRY: Caught on, have you? Took you long enough.
RON: I wasn't the only one who thought you'd done it. Everyone was saying it behind your back.
HARRY: Brilliant. That makes me feel loads better.
RON: Least I warned you about the dragons.
HARRY: Hagrid warned me about the dragons.
RON: No, no, no, I did. No, don't you remember? I told Hermione to tell you that Seamus told me that Parvati told Dean that Hagrid was looking for you. Seamus never actually told me anything, so it was really me all along. I thought we'd be all right, you know, after you'd figured that out.
HARRY: Who could possibly figure that out? That's completely mental.
So, yeah, apparently Ron had thought he'd make things right by making sure Harry went to see the dragons with Hagrid, but the delivery was, shall we say, a little bit muddled.
In addition to the stuff with Harry, Ron had some conflict going with Hermione as well. Hmm, come to think of it, Ron was actually the movie's biggest drama queen (well, other than Voldemort).
Ron doesn't really seem that aware of why he's stirring the pot with Hermione, but he's definitely doing it. When he discovers that Hermione is going out with Krum, instead of being neutral or even supportive of his friend, he's super negative about the match:
RON: He's using you.
HERMIONE: How dare you? Besides, l can take care of myself.
RON: I doubt it. He's way too old.
HERMIONE: What?! What? That's what you think?
RON: Yeah, that's what I think.
Krum was Ron's hero until he started going out with Hermione, so it doesn't make a whole lot of sense that he'd suddenly be trashing him and accusing him of using Hermione.
But Hermione gets it (see, she's so smart): she knows that Ron's attitude is totally the result of jealousy. So, she lays it out for him:
HERMIONE: You know the solution then, don't you?
RON: Go on.
HERMIONE: Next time there's a ball, pluck up the courage and ask me before somebody else does! And not as a last resort.
Ron doesn't get where she's going with that, though…or, at least, he pretends not to:
RON: Well, that's…I mean, that's just completely off the point.
It seems that he's not quite ready to admit his feelings yet, but the message is clear (well, at least to the audience): Ron's crushing on Hermione. Now, we'll just have to wait to see how long it takes fro him to actually admit it.