Bent GMC

I decided to get a transmission, service flush, etc. and when I got the truck back two days later, I get a message that engine oil is low, so I check it and sure enough I’m 3 quarts low. I called emailed regarding maybe an apprentice inadvertently drain the wrong fluid. Got no response. Issue I’m having is after I filled my oil back up a month later I had to add two quarts and then a month later which is now adding 3 quarts. I have absolutely no leaks of oil anywhere on this truck so where is this oil going? Is it because of the switch to four-cylinder or because I got a transmission service? I don’t know what the heck is going on, but I’d like some advice.