Why does my check engine light flashes?

If it was a cam sensor the vehicle would not run at all it would have died out and left you stranded. The shop sounds Shady To me. all mechanic shops should have a diagnostic scan tool. This is pretty much a must-have tool for today's vehicles. They should be able to plug it in and within a matter of seconds be able to bring up the vehicles codes, defining each specific one. If this is done properly, the vehicle's onboard computer will show u exactly the problems it's having. That's the starting point to troubleshoot your specific problem. Service engine light flashing on and off but not staying on can represent many things. some common examples: gas cap not tightened completely, O2 sensor acting up (there's more than one), early signs of transmission failure ( slipping/stalling in specific gears). The best help that I may offer to you is find a new mechanic... this one either doesn't know / doesn't care about you. just after ur money

It is FLASHING , actually , to alert you that the code(s) that are set (that would normally only be indicated by the light being on solid) , are likely doing damage to the catalyst system and engine. Highly recommend you stop driving it until it can be checked out.