Regardless, a vet visit is in order in my opinion. A friend of mine had a cat who had tummy hair loss that looked just like what you described. She told me the vet said the she was "stressed" about something and took to licking her tummy "obsessively" as some kind of comfort reaction to whatever was causing her stress. They had recently moved to a new house, and this behavior started within a month of that event, and it did subside over time.

If I remember, the vet said they could prescribe a kind of 'kitty prosac' but to see if the situation improved over time.

Is there anything new in the kitty's life?

I remember when Ritz was going through "puberty", right before we had her spayed, she got a mild urinary tract infection that the doctor said is common with the stress brought about by going through changes like that, and that cats, like people, react to stress in different ways, including changes in health and habits.