The lorry driver was taken to clinic to be looked at as a precautionary measure.
Dr Amer Hamed who was in a vehicle behind helped a GP and other medics tend to the injured woman before paramedics arrived six minutes later and rushed her to hospital.
A force spokesperson said: "A multi-agency operation is under way on the M5 in South Gloucestershire following a serious collision".
Witnesses reported seeing people lying by the side of the road as members of the public helped pull survivors from wrecked cars, with some performing first aid.
Huge queues built up behind the crash scene after the motorway was closed in both directions. One was absolutely destroyed and another had flipped over, according an eyewitness.
"A few people offered us water and one man left his auto to give nourishment".
Pictures from the scene showed a truck on an embankment, with the rear of the lorry sticking up in the air.
Police were called at just after 2.30pm to the site between J15 and J14 northbound.
Emergency services rushed to the seen after a lorry crashed through a central reservation from the southbound carriageway. "Multiple #EmergencyServices #pray for people involved".
Expressways England cautioned of "long postponements" and prior said there was a six-mile line northbound between intersection 18 for Avonmouth and 16 for Almondsbury.
A statement read: "Road users further afield intending to travel through this area should consider alternative routes and allow plenty of extra time for their journey".
Highways England said the "early expectation" is that the M5 "will remain closed in both directions late into this evening".