it would take more than a jack to bend the sub frame more like to much air in the tyres also got to let the tryes bed in
 
You can jack cars safely on the rear beam, some mot testers put the beam on the rear shocker mounts with a block of wood which is fine, why didnt they just drive onto a block of wood and use the sill? Anyway check the tyre pressure, also check if its rotational it could be on the wrong way.