The chances are low but there is always a chance. It's worth getting a test anyways in case you picked something else up instead.
Have you been assessed on the HIV Prevention Forum? That should be your first stop.
Three ways that HIV can be transmitted
1) UNPROTECTED INSERTIVE ANAL/VAGINAL SEX
2) SHARING DRUG WORKS (NEEDLES)
3) MOTHER TO UNBORN CHILD
No matter that your penis was not fully erect......no matter that it was only 5 seconds. What you had was unprotected, insertive vaginal sex and you should test at 12 weeks for a 100% conclusive result for HIV and sooner for other STDs.
Please double check my response with Teak or Lizzie Lou on the HIV Prevention Forum.
We have an HIV forum here at Medhelp. You may want to post your question there. The people are very knowledgeable and will let you know if there is any chance of you getting an STD from what happened.