I am sure everything is fine, but if you are very worried you can always call your OB's office. Maybe take kick counts to ease your mind, and stay cool!
It should be fine. It wasn't too long. I'd ask your dr about it tell them you want to use it but is it safe if you fall asleep an end up w it on your tummy in the night. I know many drs suggest heating pads to help w back aches in pregnancy