Lack of power I hope you can give me advice

Good day. I hope you can give me advice, as I am still waiting to hear from my mechanic and scared to drive my car. I recently had my engine rebuilt/overhauled. (Got my car back three weeks ago). Mechanic stated to bring car back for oil change after about 600 miles, which was last week, but could not make an appointment due to time restraints. Did an additional about 300 miles and then she started having major lack of power on acceleration from yesterday. Have to do quite a few uphills daily to work and back and struggling even in third gear. Need to use car later today ( in about six hours) to get home and still no word from the mechanic. What could be the problem and can it be due to the oil change not done after 600 miles? Can there be damage due to that after only that? I need to know if I can still drive my car for a day or two. Please help. Did check oil this morning and the attendant said it needed only a pint. After the oil was added, there was some white smoke, but that could be due to the pint of oil added? Please help.