Spotting for cuckoo

My 1997 Ford Escort misfired, Need to find out if this is serious and what can cause it.? - summer escort

I overtook a car on the street the other day and the motor spikes. The next thing I remember is the light of my script Check Engine Light. I have my brother because he has a code reader OBD II. It was this code Detected P0303 Cylinder 3 Misfire
The strange thing is that I only changed the spark plugs and son in this summer for her. What would have caused, and it is serious?
FYI, my car has 103.000 miles on this topic. Thank you for your help.