A long one but I need some advice 😫

My 3 year old went 2 weeks without going to the toilet (number 2) previously they went through a phase of not wanting to go because it hurt due to constipation, this resolved fairly quickly thankfully and they were going to the toilet with ease. Fast forward to about 9 months later and they went the 2 weeks without going to the toilet, so we took them to the doctors, not showing any signs of wanting to go and not in any pain they prescribed movicol and lactulose, we used this for 2 days and they started to go to the toilet so we stopped using the medication, it’s now been a week and the toileting hasn’t stopped, to the point they don’t actually know they need to go and they’re soiling themselves, it’s not a whole ‘poo’ it’s more like a ‘shart’ or a bit more, but they’ve got no awareness they’ve been and they’re now extremely sore down there. How on earth can we stop this? Fully potty trained for a year and still takes themselves to the toilet throughout the day. I’m tempted to use some dioralyte or take them back to the doctors but it feels strange to go to the doctors for one thing and then the complete opposite a week later?! All their underwear is getting ruined and we’re at our wits end with it. Any advice please?! And any questions if I’ve missed something out let me know and I’ll answer! 😫😫