Raw patch after flea-ing?

[BuggersMuddle]

BuggersMoggy has a big raw patch on his back. We gave him his usual flea treatment a couple of days ago and he seemed fine (tbh is still fine), but I picked him up this evening an noticed a diamond shaped patch of furless skin about 1" long by 1/2" wide (at widest). It looks a wee bit oozy / red and raised, but no signs of infection.

It was Frontline and I have heard that there have been some recent issues, but don't know much above anecdote.

He's not showing obvious signs of brawling. He has been catching bloody rats, but that's pretty much his modus operandi these days. He is a bit vocal / grumpy, but we've also been re-organising stuff in the house (significantly) and he is a fairly vocal cat.

Just wondering if -

(a) I need to be concerned
(b) Anyone knows about the Frontline issues

I'm was thinking wait and see, but worried about ooziness and the chance of infection. It of course doesn't help that the building site is still there and is his favourite stomping ground, so could be lime / cement and entirely coincidental to us administering flea treatment.

[cozietoesie]

Is it at the site of the application?

[BuggersMuddle]

It's in the same area, yes. He's never had a problem before and it came from the same packet as others we've used.