brand new macbook pro randomly going into sleep mode when using the battery. starts up again when i connect the charger. battery issue? or blocked fans?

baught macbook pro from tesco in the uk and at first worked fine. twice today though its gone into sleep mode randomly while i was in the middle of doing something. i understand there is a thermal cut off but i dont see how it would be that as it has run at a hotter temperature to touch and has never done this... i have only had this for 3 weeks roughly but today i also did not have it on a flat surface like i normally do it was directly on my lap... but still not that warm..