I'm sure you're worried, but I think assuming there is an "HIV threat" is a little preemptive/alarmist on your part. And maybe insinuating something not very nice about the provider you chose.

Get tested and see what's up! Knowledge is power!

GYT! Weird she would reuse the condom.

You can never use to many condoms.  Odd to say the very least.

Odd that she didn't use a new condom

Very unprofessional of her to turn the condom inside out. If she is that cheap and unprofessional I would get tested and the full panel at that.

My doctor knows what I do.  I have had condoms slip off 2x in my career.  She said the amount of exposure to the HIV virus with just a slipped condom with no finishing bare is very small.  I still test every three months, but HIV risk is small from a brief slip (unless you are doing anal and it slips).  

What IS a big risk with even a small exposure is chylmidia and ghonnarea.  It can take a week for enough bacteria to grow to even test positive if you did pick it up.  Both times it slipped, my doc perscribed me 1000 mg of Zithro as a precaution, which cures both of those bacterial STD's.  If you have an open relationship with your doctor, you may be able to explain what happened and ask your doc to perscribe Zithro as a precaution. I now keep 1000 mg of Zithro on hand at all times, so if it ever happens again, I can just take the pills and be safe.

Good luck.
- xoxo Hannah