This means that somebody was too loud during sex the night before.
'Microwave' refers to the coitus that was had.
Justine: Good morning, guys! How did you sleep?
Carla: Not very well.
Justine: why?
Carla: I heard you using the microwave last night..
Justine: Oh dear..
'You're just jealous because you heard HER using the microwave but you can't handle that heat!'