Your OB seems to be on the right track. It does not matter which progesterone preparation you are taking, my usual choice is prometrium 200 mg intravaginally 3 times a day, but there is nothing wrong with intramuscular progesterone (except that it hurts and I do not think it is any better than intravaginal), crinone 8% once a day intravaginally, or intravaginal progesterone suppositories. I usually ask my patients to test for ovulation with urine ovulation prediction kits, have intercourse the day the urine first tests positive and the day after, and start progesterone 3 days after urine first turns positive. Since progesterone can sometimes prolong a period, I recommend a pregnancy test on day 31 or 32 if you have not yet had a period. If the pregnancy test is positive, stay on the progesterone. If negative, start the progesterone, wait for a period, and repeat.
Finally, if the spotting continues this month even on progesterone, your ObGyn might want to consider doing a saline ultrasound to check your uterine cavity and make sure you do not have a small fibroid or polyp in the cavity.