When I was serving as an elder, before ever making a call on any JW by two elders, it would be discussed by the elder body, then two brothers assigned and these two brothers would call, both on the phone line, not just one, but both on the phone line at the same time, to arrange for a scheduled time, especially so if it was a DF'ed person, and even more emphatically so if it was a female.
That they showed up un-annouced, is very suspicious. Not to make you paranoid or anything, but since you just saw them at the Hall three days ago this stinks of an "investigatory visit".
You did well not to see them in. If you invite them you are giving in to their tactics. Next time just tell them you're in the middle of getting dressed since you just came out of the shower and have an important appointment at school you must go to. (Their fantasies will run wild).
If it happens again tell them they just caught you coming out of the shower again, and follow this up with the question "Do you guys plan this? 'Cause you always seem to know when I'm naked and getting out of the shower. If I didn't know better I'd feel like I was being stalked!"
Then at the very next meeting ask another elder if you can ask something in private with him and another brother and say that you would appreciate if they're going to call on you that they make plans ahead of time because twice now they've shown up at your door when you've just come out of the shower.
It will get back to the whole body of elders, you better believe it, and if they do try to visit again without an appointment, then sure enough this third time is highly suspicious, no really, the second time may have been a coincidence but now you're getting awful worried and will be calling the CO.