I’d imagine it’s your stitches healing...? I’ve never been stitched but I have torn, and the stinging wasn’t just with the initial wee.
Keep the area clean and dry, and lean forward when you wee (assuming the stitches are at the back, I tore frontally but they didn’t tell me and I didn’t think).
Ring your midwife if you’re worried about infection or anything though, they won’t mind.