NOT A TEACHER
(1) You have asked a wonderful question. No, I CANNOT give you the
reason, but I CAN offer a few thoughts.
(a) Many native speakers here in the States also get confused by
those three verbs: To lie (not tell the truth), to lie (be on your back),
to lay (something).
(i) Sometimes I will hear even TV newsreaders say: "The victim was
laying in the street"!!! (Instead of "lying.")
(ii) The most common blunder may be "I laid on the beach yesterday"
instead of the correct "I lay on the beach yesterday." I have a theory.
It is probably wrong, but I am delighted to share it with you. I think that
native speakers feel that "lay" (the past of "lie" on your back) does not
sound like a past tense. As you know, many verbs have a past tense
that ends in -ed. So speakers get accustomed to the -ed sound.
Therefore, they might think that "laid" sounds like the right past tense
because it has the "d" sound.
(2) Of course, "I am laying in bed" is very bad English. Remember that
"to lay" is transitive. That is, it needs an object: I am laying the flowers
on the table. In your sentence, you want to announce that you are
currently on your back in bed: I am lying in bed.
(3) Don't feel bad. I am pretty sure that most of my fellow Americans
would fail a test on these three words, and I doubt that I could get a
100% mark on such a test.
Student or Learner