Scab like bump on my cats head?

My baby girl has fleas and I treated her for them already and am unable to treat again for about 3 more weeks.
Today I noticed she had gotten them back again and therr is a scab like bump on the back of her head!
She is an indoor cat and I havent a clue as to how she has caught these damned bugs and I am really worried about thia scab like bump!
Could this be from the fleas/itching or something else?

My guess is it's an allergic reaction to a flea bite.

Should I take uer to the vet to get it looked at?