I will try to clarify your doubts as much as possible. An rtpcr negative but ct scan diagnosed covid ( that too with ct score 2 ) can be something else which is not covid. Many viral pneumonias can mimic covid pneumonia. Your current symptoms can be covid. Only way to confirm is by an rtpcr test. Few other details are required, like if you have any fever, what your o2 saturation is etc. The pain that you complain of, can be signs of diaphragmatic exertion or even pneumonia involving base of lung. There are no blood tests in my knowledge that diagnose gastritis, it is a clinical diagnosis in general and there are invasive tests for some special cases. Although technically covid recurring after 3 weeks is not very plausible, but "Long Covid " is an accepted disease where covid infection can persist for months. Also, as i said earlier, this might be your 1st covid infection and the previous one might have been some other viral pneumonia.

Highly unlikely that it is covid symptoms are typical of gastric reflux... Take pan d or any other antacid before meal And avoid spicy food