Swimming for 2 months, now mild pain in pecs for 2 weeks, I didn't rest, could it be an injury? Or is it respiartory muscles? Btw I have mild scoliosis.

Overuse/Strength? Most likely it's a little overuse inflammation from doing an exercise your pectoral muscles are not used to, especially if you have not rested very much. Could also just be some soreness from working out the pectorals - to get muscles stronger usually involves getting muscles sore. Mild scoliosis should not matter. Your respiratory muscles will strengthen with swimming.