Gosh I dont' think you're negative at all! I would have done the same. Especially being 20 mins each way, you'd have to be gone for at least an hour.
I leave DD 10 home for 1/2 hour while I pop to the shop. It is 2 mins away and she usually calls me while Im out!!! (cute). But I wouldn't leave her if I was going any further.
I can't see that being responsible could be taken as negative. MIL's can be soooo frustrating. She's prob annoyed because she came across looking so uncaring!! xo