Possibility of angina pectoris

According to the data you provided (chest pain in the middle and sometimes to the left in seventh week of pregnancy, ST depression on ECG) it is possible that you might have angina pectoris.

Angina pectoris is a chest pain related to the function of the heart. The heart needs enough oxygen in order to work properly. In case the supplies are not enough (the very heart cannot pump or the body needs more blood to be pumped-when under physical stress), pain might appear in the area behind the chest bone. The pain might irradiate in the left part of the chest and to the left shoulder. ST depression is actually the ECG manifestation of ischemia (a condition in which not enough oxygen is supplied to the heart muscle).

The reason that you did not have this condition with your first pregnancy might be simply that your heart was not in that condition previously.
You might want to visit a cardiologist for a physical examination, and 24-hour ECG Holter monitoring which would tell whether there are ischemic episodes and when they occur (under which conditions: physical or metal stress, or part of the day).