Odd foot blister

[Westjt]

Monday night, I felt what I thought was a mosquito bite on the side of my right foot, I was sitting at a dinner table so I just moved my foot out of my shoe and scratched it with the other foot, not looking at it. It continued to itch and when I looked at it, I noticed a blister. I went home and examined the blister and ended up popping it. It mad a lesion/ulcer, not sure the term, but I was not a superficial blister. It was a bit sore to the touch but it began to scab quickly and now has a yellow scab layer. What concerns me is the way the blister came about. Seemingly randomly and itchy like a big bite. I had been walking with shoes without socks but the shoes aren't new and quiet loose fitting. Anyone have any ideas of what this could be? Is there cause for concern?