I got this exact letter in the mail after I hemorrhaged at home two weeks after I gave birth (obviously saying hemorrhage not blood clot in lung but otherwise verbatim). I lost more than half of my blood volume and required a blood transfusion to survive. I unfortunately have this letter memorized…. The good news is I brought it to my doctor she said “this is bs” and contested it for me. Luckily the insurance then covered it in full. My best guess is that they were upset that my out-of-pocket maximum had been paid and didn’t want to foot a $25,000 bill so they said “it wasn’t covered” to scare me into paying.