Welcome to the forum @salemn99 Unfortunately we can’t diagnose on here but can I suggest ringing 111 and asking them for advice but definitely contact you GP tomorrow morning. The sooner you get it diagnosed and treated the better as you don’t want it to become infected.
Leg ulcers can start in many different ways.
If you think you may have the start of one, please make sure you get seen right away.
If you can't reach your GP, ring 111 for advice and let them know you fear you may have a diabetic leg ulcer.
Salemn, your question is a bit like asking how a cold starts. There might be some usual symptoms, but nothing is nailed on.
My strong steer to you is if you have concerns, speak to a health care professional about it, sooner, rather than later. Don't wait and see what happens. It makes sense to begin treatment or even reviews as soon as possible.
Diabetic ulcers can become very nasty indeed, very quickly indeed, with potentially catastrophic, life changing consequences. Please, please don't think I'm just trying to scare you. I'm not, but trying to help you understand how serious things can be.
Fingers crossed your concerns are nothing to worry about, but it would be good for you to know.