My car was running beautifully, had a oil change

my car was running beautifully, had a oil change and filters change, again drove beautifully for 3 days, then overnight, it starts great, but shakes between 5mph-25 mph, and is sluggish when i take off from a stop. once i am going it seems fine. i have a 2004 chevy cavilier, keep up the oil changes every 4 months, etc. This problem started overnight and am beginning not to trust the mechanic . I put in gas the night before. since then i have went to another gas station and filled up the tank and put engine cleaner in the gas. This has improved the problem alot, but the check engine light flashes still, and still shakes, not as bad, only at stops and take off, then it is sluggish until i get to 30 mph and higher. on the highway it drives good. But is still afraid. some days the shaking is not so bad and the sluggishness is not so bad, then the next day it starts up again. i just do not trust mechanics. i went from driving beautiful to this problem overnight, no symptoms, no warning.