laid is the past tense of the active verb lay in English, but that verb means lägga (which has the past lade) in Swedish.
The stative verb lie has the past tense lay in English.
So stative lie -> lay = were lying = låg (present: ligger)
but active lay -> laid = were laying = lade (present: lägger)
Kinda tricky and many native speakers of English seem to struggle with this.
I think this article is pretty good: http://www.quickanddirtytips.com/education/grammar/lay-versus-lie
It's not strictly correct, but if it's clear that you are/were inside, then it's easily understood that you mean the floor. Ground can function as sortof a catch-all term for whatever surface is underfoot, though floor would certainly be more technically correct.
An interesting term I just thought of as a result: the ground floor of a building is the floor that is level with the ground.
We have other threads where one or two people claim to use it that way, with many more others saying that it's definitely wrong. I think it's a situation where we won't be able to please everyone - but it also seems that using it in this way is not very common, and not idiomatic enough to be accepted.