My girlfriend quit smoking for two weeks ago but she is experiencing tightness in her chest. Is this common?

?withdrawal. Look at the cartoon for some causes of chest pain. See your doctor for evaluation treatment varies with the cause quitting smoking should not cause chest tightness. If you think she may be having withdrawal symptoms, you may use nicotine patch. Consult a physician.
Possibly. Nicotine withdrawal is very common in smokers who quit smoking "cold turkey". They experience all kind of unpleasant physical and emotional symptoms. Although, chest tightness is probably not the most common one, i guess, it is potential to have some unpleasant sensation in the chest, however, she will need to watch for other symptoms - cough, phlegm production, painful respiration, fever etc.