I got my stratocaster set up about 1-2 weeks ago and ever since i got it back ive had a miserable time trying to keep it in tune. I had switched from 9s to 10s and had it set so the bridge was flush to the body which would "help keep it in tune better." the g string has been going sharp ever since i got it back almost immediately after tuning again to the point where its barely playable. i thought it might just be an issue with the string but the last couple days all 3 of the high strings have been going sharp on their own.

im thinking maybe they tightened the strings too tight? i would like to try to get this squared away without taking it back so if anyone has any suggestions they would be greatly appreciated
I've had similar problems with guitars that I've owned, particularly with the three highest strings. I found that the strings were binding in the nut. I used a little graphite under the strings in the nut and it fixed the problem. It comes back every once in a while, but I put some more graphite in there and it goes away. Hope it helps.