Ahhh don't worry lah, mama, accidentally lying on a warm (not hot!) heating pad for a bit won't harm baby, but since you're already feeling anxious about it, just mention it to your doctor at your next check-up or give them a quick call if it'll put your mind at ease, okay? A few of us mummies have had similar panics and our gynae always said as long as it's not hot hot, it's fine, your body also protects baby quite a lot from temperature changes. Next time maybe try putting the heating pad on your side instead, or tuck a pillow between you and it so it's not direct contact, especially if you're worried about falling asleep with it!