I am at day 36 LMP. Ultrasound showing only very small sac. Doc said it looked like 4 weeks. HCG levels are at 15000 already. Is it possible I am pregnant with more than 1?

Possible, but often two gestational sacs will be seen even at 4 weeks. A one time hcg level does not tell a great deal so your doctor will most definitely want another scan, and likely more blood work, within a couple of weeks.