I had a collapsed lung 3 weeks ago, now again I have the pain in my back that I had before treatment, can it be the lung collapsing again, thank you?

Could be. Are you having trouble breathing, if so you may have a recurrence of spontaneous pneumothorax. Visit your nearest emergency room and enroll in programs to help you quit smoking. As long your you smoke , you will always be at risk for spontaneous pneumothoraces.