Is it common for GERD to get worse after a nissen surgery before it gets better?

Not usually. Classic gerd symptoms like heartburn usually improve immediately. However with the swelling some symptoms are made worse like trouble swallowing. This usually improves in a few weeks.
No. Usually gerd should get better after surgery, but the nissen only works after you eat ... With an empty stomach, the gerd would be the same ...
No. The most common manifestation of gerd is heartburn:this should improve almost immediately after surgery and never get worse. Other symptoms such as difficulty swallowing (dysphagia) can temporarily worsen after surgery but not for more than a few weeks. Please let your surgeon know about these symptoms.