Here is the problem in trying to answer your question. A complete lack of data that has any credibility and ability to verify using a key board and a monitor to read what is keyed in.
You say something changed and while I am not condemning you for saying it, the manner in which it was said was not a logical way to go about saying it. It made no sense. I can only sit here and question what you will be able to understand even if I had something technical I could offer you.
You pretty much pay some one to check your car putting you and your car at the mercy of the credibility of the repair agency. I do most of my own work. I do have places I go to in order to have things done that I do not have the tools for. But on one trip the counter man gave a lady having brakes put on her car at an exorbitant cost a line of "Bull" about how wonderful the job was done and the quality of the workman ship performed and she ate it up like a hungry lap dog. While I am listening to this thinking "That dog don't hunt!"
That is the mark of a salesman learning to say what they wanted to hear.
I offered to go through my neighbors car having looked at it a bit and if it needed anything at all, parts cost would have been well under a C note. I was not trying to "SELL" anything, but instead he spent over a grand to have questionable or at best mediocre work performed only to have variations of the same problem a bit later. Last week I said something to another neighbor about some problems caused by his own neglect and his response was "Don' t talk to me like I don't know what I am doing". I went home, he was clueless, blatantly so, he had no idea what he was doing. The car still sits there broken.
Not my problem. I went home. I maintain my fleet of 5 operating cars plus 2 under restoration on a next to nothing budget. I am not a salesman and yes I am venting a bit. Sorry.
Your fuel usage does not sound unreasonable under the conditions at the moment. Todays fuel injected computer controlled automobiles are actually technological marvels and when correct make adjustments to them selves that until the the technology came along to do it automatically some where in the late 1970's it was done manually, routinely and as needed.
In today's world that kind of repair is substituted with pretty much get a computer scan of the error codes registered in the car. That is the starting point. But you will pay for that privilege. And the cost of that might very well justify buying your self a scan tool that will read OBDII computer logic which will probably be around for quite a while so it will be useful for a long time. I don't have one yet as I have not needed to know anything from a car using OBDII logic. OBDI logic could be obtained by using a stepped sequence in the car itself and count the flashes from a blinking check engine light. It was rudimentary but adequate so far for things I have needed. I now have a professional level scan tool that will let me read and operate the computer on the two cars I own with OBDI or Pre OBDI which my Saturn is. Through the kindness of a friend who bought a new one rather than update at $1000.00 a year the needed updates. Yes that is expensive. A new hobby level scan tool can be had for about $200.00 or less depending on the features. Having some one scan and tell you what is going on may cost up to $100.00 so for an extra C note and about $40.00 for a book for your car from an O'Rielly's auto parts store and reading and using them might be able to get you started in actually doing some of your own work which would bring both satisfaction and a whole lot cheaper to do things your self.
Get an error code read out some how.
If there are none, than there is no real problem you can deal with.
If there is something than report them and this forum can go from there.
Remember this while contemplating your next step (one of which is to get error codes checked) why pay some one to screw up your car when you can screw it up yourself cheaper!