Progesterone Level

Hi I am 6 weeks pregnant today from an Iui and yesterday at 5 weeks 6 days we had an ultrasound and heard the heartbeat. Baby was also measuring well. However, later my dr office called and said my progesterone level was a little low at 17 even though I have been on 100 mg of endometrium. I am going to triple the dosage but should I be worried? With my twin pregnancy i was 42 at 6 weeks 1 day so this seems much different.