yo help me tech problems

[Dominic22]

k i got a 01 audi a4 quattro

upgraded turbo and intercooler and exhaust and other shit but still stock down pipe

and today i was doing a mountain run and i down shifted to

3rd coming around a corner and shifted back up to 4th but

when i did that i just heard all my air pressure release and the car died.

after i tried starting it and it starts but it boggs out under 1rm

and dies or sometimes it will idle but struggle to keep up

what might of happened i think its cause to much backpressure

but idk what it could of cause please let me know on what

damages might be done