They only say it's bad because your baby can't regulate it's own body temp, but u should b okay, youre almost due.
It's only really an issue if you raise your body temperature. A spot warmth should be fine. Your baby is basically fully developed by now anyway.
I've never heard of that being bad. Anytime I mention pains to my doctor she tells me to use a heating pad to help. As long as you didn't let it burn you, I'm sure it's ok. But ask your doctor if you are worried