Why is my breathing bad when I stand or sit up

[WorriedAnimeFreak]

For a long while now I think since early 2021 i been having bad breathing trouble ,  I noticed every time I stand and continue to stand it gets worse and worse makes me feel like im going to panic,  I tried to see the doctor about it, he tried to listen to my lungs and he said it sounds fine but if im still having it we could schedule breathing test at the hospital , So I finally got a hospital test done on testing my lungs if they were working fine and the doctor told me it looks normal so he dont know why im having it.  He is just a clinic doctor.  But to this day im still having horrible breathing feeling like its very thick air like a thick filter for air going through my body, it just feels slow thick and like im not getting enough oxygen but getting some air,  its best how i can describe it.  I do know personally I have bad anxiety not sure if this really can make all this feel like its happening,  because I feel sick like every single day as feel and im finally sitting down again and feeling a tiny bit better but still feels slow to breathe in,  I am underweight a lot, i dont work out, I am mainly in my room,  so I am out of shape so maybe if thats doing it too... I dont know I just need some answers to help calm my nerves.. If the doctor cant find it im reaching out online here make you all might know.