Can a c-section incision open up after 3months? Never had any infections just started walking / running and I see it pink sometimes & stings but dry

Unlikely. A significant amount of healing occurs within 3 months after a C-section. As long as there were no complications after surgery and you had an otherwise normal recovery, you should be ok. The stinging and pink coloration can occur for several more months. If you continue to have concerns, have your doctor evaluate to make sure everything is still ok.