I worry about anything like that with my feet. One of the first weeks I ran a lot (33 miles...), it was the middle of winter and pretty cold, so I was wearing 2 pairs of socks, not thinking that much about it, and one of my toenails turned black and it was pink and puffy around the edge so I went to the immediate care place, the doc assured me that she was very glad I'd come in, rx'ed some sort of what she said were strong antibiotics and sent me to the podiatrist, who took the nail off and pronounced it ok. Both docs assured me that it was very reasonable to get stuff checked out as if you don't and you have problems, it can get out of hand, or foot, quickly I think?
See a doctor immediately! Don't take any chances.
It doesn't look infected from your photo, but I'd have it looked at because things can quickly develop. Don't risk it. Any foot sore is cause for caution & medical attention.
I don't know what qualifies as an official ulcer, but any open type foot wound should be examined. It looks like a raw spot where the skin was rubbed.
I learned the hard way, what turned out to be a TINY little cut on my big toe turned into a 3 day hospital stay with a bad case of cellulitis and IV antibiotics. It doesn't look from your pic infected or anything BUT I'd still get it checked out cause these foot things can turn bad FAST. Your feet take so much abuse, it doesn't take long for a tiny small little sore to get BAD.