Bath tubs are perfectly fine as long as you can climb in without flinching. I lived in the bath for almost two months to help my nausea and it was fine. It can completely cover you as well, as long as it's not scalding hot.
just to add to this question... is taking a bubble bath at home with warm water that doesnt completely cover your body dangerous as well?? i tend to do this all the time when i get home from work and it helps me relax.. i am 16wks and since i have heard about the hot tub i never let the water cover me completely and the water is usually not "hot tub" hot...
I went into the hot tub all the time when I was pregnant, my doctor said as long as the water was under 100 degrees it was fine. I think you will be fine and the baby as well. How hot was the water? if your unsure go to the doctor like nickie had suggested. anytime your not sure, and questioning something always go to the doctor for reasurrance.
nickie is correct about the reason why it is advised against a pregnant women using a hot tub. That said, it is HIGHLY unlikely that this would have caused any problem for your baby due to the short length of time your were in the tub and the fact it was not very hot.
Doctors advise against using a hot tub because it can cause the amniotic water to get too hot for the baby. Also, another reason they advise against it is because there really haven't been enough studies done to say either way. I would say you need to go to your doctor, explain the situation, and let them look you and baby over to see if anything was damaged. Good luck!