It's okay since you don't know and still in the 1st trimester but from now on avoid using any medicated plaster. Use hot/cold compressor instead. Can do massage when you've reach your 20 weeks mark.
asked my gynae before and he said okay to use once in a while, but advised not to use everyday. since you only used once should be fine. dont worry!
I am not sure about that. You can use warm or ice pack to ease the back pain.