Transit through Kunming to Shanghai

I flew from Kolkata to Kumning, expecting to then fly on to Shanghai for a three day working visit, having been told one could have a 72 or 144 hour via in Shanghai. Immigration told me as I has entered China via Kunming, I only has 24 hours in China and had to change my return flights from Shanghai departing after 72 hours to ones that depart within 24, was this correct, as I had to fly to Shanghai to then almost immediately return to the u.k.