Originally Posted by
vashti 
Oops, missed that...
At 25, just tell her you are embarrassed she caught you in an intimate moment, and that it wasn't what it looked like. Then drop it. You aren't responsible for her suspicious nature.
Exactly what Vash said. The more matter-of-fact you are about it and calm, the more she will be reassured. Also, it will make her feel silly if she chooses to make a big deal about it.
Next time, lock the door.