Up to something !!!

JenniferYeah, great. But who recorded these for you?
BillyMy classmate, as a joke. It makes me sound like a womanizer.
JenniferWell, then let me record one for you.
BillyOK. Cool. Be sexy. Here. Go!
JenniferHey, jerk! Answer the phone, you idiot!
BillyHey! I should have known you were up to something.
Explanation :
If you say that someone is up to something, you mean that they are secretly doing smething that they should not be doing.
Example :
Why did you need a room unless you were up to something.