cat hair loss

[anitacurnin]

my 3 yr old moggie regularly cleans herself, we treat with the back of the neck spot on flea stuff once a month, but she's developed a bald spot on the base of her tail/bottom of her back.she does scratch a bit, but never the area where she is losing hair.
My husb is paranoid that she has fleas, he says he's been bitten, but I think it's just a heat bump, i've never seen fleas & religiously treat her.
Any suggestions?