Rear derailleur bouncing up and down in easiest gear I crossing at a cross junction, so I literally only had a few seconds
  to cross because the cars beside me would turn right. As I pushed
  towards the curb I heard a loud bang.
  
  Anyway after that incident the rear derailleur now bouncing up and
  down while I pedal in the lowest gear. Only the easiest gear though.
  Otherwise it's shifting up and down the cassette fine.
  
  what did I do?


It's a shimano tourney on a this bike
https://www.cyclesurgery.com/p/orbea-carpe-40-2018-Q3414421.html?colour=122

I know at least one of the springs is intact because when I push the derailleur inwards, it springs back immediately after I let go. 

with the rear wheel out, I can feel there is tension on the cage wheni pull on it
That sounds like either the A or B tension spring on the derailleur broke. 

The A spring rotates the cage to tension the chain. The B spring rotates the entire derailleur around the mounting bolt. If you pop the chain off the chainring on the crank to slacken the chain you should be able to see that the derailleur is moving freely without spring tension. 

The solution is to replace the whole derailleur, as I don’t believe they can be disassembled to replace a spring.