In short:

One evening, Rodrick, probably drunk, called you. Since you were exhausted and kept dozing off for a few seconds at a time, you didn't hang up. Apparently, you said to Rodrick, "I love you too," just like he keeps repeating. You don't even remember him confessing any feelings, let alone telling him you loved him. You don't love him. Him? You'd rather die.

So there's no other explanation than that he's lying.

Since then, however, Rodrick thinks you're in love.

Like I said, he thinks you're in love and is now treating you as if you were in a relationship. You're confused and trying to get rid of him. Sure, constant arguing and fighting was exhausting at times, but being in an unwanted relationship with him? It's just nerve-wracking. No matter how often you tell him you definitely don't love him, or whatever, he won't leave.

You're walking through the mall when someone—well, not just someone—Rodrick puts his arm around you.

"So, what's up, sweetheart?" he asks, kissing you on the cheek.