Had a cavity filled 3 months ago,dentist suggested that rc might required,theres not much pain while chewing &gums are fine too but it hurts while flossing &the molar is still black from one side,im worried abt infection. should i go with xray & rct?

Pain with flossing can be caused by tooth decay or infection. Yes, you should visit your dentist again.

This is a general information site, not a diagnosis/treatment plan site. pain on flossing is an indicator of, among other things, infection. An examination is required. See your General Dentist or a specialist Endodontist for appropriate testing and treatment plan.