Been out for a year, low blood pressure

So this morning I went back to the gym after a year of no frequent exercise. I'm quite frankly shocked and embarrassed at my level of fitness right now, but that will get better. The thing I'm worried about most is my blood pressure. The trainer took my blood pressure before and after a workout. Before I was within the normal systolic and diastolic readings (I was 124/82), but after the workout my systolic went down to 70 something! I did feel very weak and sick. Will this get better with my fitness? Since I last went to the gym a year back K've gained 40lbs. I'm no longer built like a lamp post but **** me it's done something hellish to my body. But yeah, any insight into the blood pressure will be greatly appreciate.