I need answers about my back pain please help!!! :(?

i injured myself at wk lifting a box of 2ltr coke n the bottom fell out as i tried to stop them from fallin ive hurt my back, i went back to wk thinkin that i wld be ok but it kept getting worse i made a docs appt but cldnt go so i went back 2 wk n pulled my shoulder trying to bring the airhose in (i was being careful as i had already hurt myself) ummm ive had an xray n that came back normal n the physio i went to couldnt find anything n pretty much said that its all in my head the pain only gets worse jus as i am able to try n deal with the pain n live life abit more normaly it seems to get worse i dnt actaly hav 2 do anythin jus laying dwn or sitting or standing for to long n it hurts which is very fustrating i went back 2 wk 2day 1 the 1st time in a few wks n im soooo very sore now i havnt broken any bones n physio didnt find anything (although they kept touching the spots where it hurts) do u hav any ideas abt whats causing the pain???? i feel like a bottle of pills i take so many even though i try had not to its soooo stupid please help me i just want to live my life normal again im only 21yo n its stopped me being able to do so much :'( any suggestions are much appreciated thank u :/