After lunch, Connie heads to the woods for a little afternoon delight (how many times can we use that joke before it gets old?). Mellors is not at the hut, so Connie takes herself on over to his.
He's eating lunch—a mutton chop and some potatoes, delish—and doesn't seem thrilled to see her. She plays housewife and makes them some tea while he eats.
After a few minutes, he gets up and shuts the door, not wanting anyone to see Lady Chatterley in his hut. Seems like a smart move, but Connie doesn't like it.
After they talk about Mellors's bad temper, Connie drops the bomb: she's heading to Venice.
He's a little pissed, although he doesn't say so. He gets especially pissed when Connie explains that part of the reason she's going is so she can pretend to have an affair, in case she gets pregnant.
Like any Men's Rights Advocate, Mellors accuses her of using him to get pregnant. She denies it, they fight, and Connie leaves without getting what she came for. So to speak.
Later that day she heads back to the clearing with the hut and they have awkward sex that Connie doesn't enjoy. We know she doesn't enjoy it because she secretly makes fun of his "poor, insignificant, moist little penis" (12.128), which is a way harsh thing to say to someone she's about to fall in love with. She cries afterwards, though, so you have to feel a little sorry for her.
Afterwards, she sobs that she can't love him—which is totally not a problem, since he's really only interested in her body at this point.
So they end up having sex again, and this time it's awesome, full of "silent amazing force" (12.145), and "ponderous, primordial tenderness" (12.146), and "dark waves rising and heaving" (12.147).
When it's over, she realizes that she does love him, after all, and begs him to say that he loves her, too. They love each other so much that they start speaking in Yorkshire dialogue—all "slai wi' me" ("sleep with me") and "appen a'Sunday" (maybe on Sunday) (12.191, 12.195).
And then comes that part that kept D.H. Lawrence from openly publishing Lady Chatterley's Lover for thirty years: Mellors teaches Connie the word "cunt."
With that lesson over, Connie runs off home to Clifford.